If there's a story that best illustrates one of the most dominant characteristics of what makes unified welterweight champion Errol Spence Jr. so great -- his competitiveness -- look no further than a 10-year-old sparring session against Floyd Mayweather Jr. 

Boxing has long been protective about bringing to light what happens behind closed doors when top fighters, who might still be working their way into elite fighting shape during training camp, focus on specific skill building in sparring sessions not meant to produce a winner -- and certainly not meant for public consumption or gossip. 

This is one of the sport's most celebrated and unwritten rules. 

That's probably why the respectful 33-year-old Spence (28-0, 22 KOs) has long been cryptic about the exact details of what went down inside the Mayweather Boxing Club in Las Vegas despite the colorful -- and, at times, seemingly apocryphal -- recollections of the many pro fighters and trainers who were present.

But whether or not it's the hype surrounding Spence's career-defining fight on Saturday against Terence Crawford (39-0, 30 KOs), in the first four-belt undisputed title fight in welterweight history, that suddenly has him being so honest about that early 2013 day, the timing is apropos. 

With a victory this weekend inside T-Mobile Arena, Spence can stand alone atop the sport's money division as the clear-cut best welterweight of the post-Mayweather era, which ended with the former pound-for-pound king's initial retirement from competitive fighting in 2015, the same year he outpointed Manny Pacquiao in a long anticipated super fight.

As the story goes, Spence was just 23 and only a handful of fights into his pro career after coming up empty in his pursuit of a medal at the 2012 London Olympics. Mayweather, meanwhile, was 36 yet still at the peak of his prime welterweight run and looking for a southpaw sparring partner ahead of pay-per-view clash against Robert Guerrero, not long after boxing's richest fighter served a two-month jail sentence for domestic battery. 

Mayweather had been entrapped in sparring controversy before when tape leaked of his 1999 session with former lightweight champion Paul Spadafora, who was six days out from a title fight and appeared to handle a version of Mayweather who wasn't in the same fighting shape. The Spence session was similar (despite the lack of video evidence) in that Mayweather admitted he was too muscle bound in his upper body after committing to 1,200 pushups per day in prison and was looking to work back into elite condition against a younger and more active southpaw.

Even as a young pro in 2012, Spence was a devastating puncher on the rise. Getty Images

While firsthand reports have long differed as to whether the black eye Mayweather sported not long after for his media day ahead of the Guerrero fight was caused by Spence, everyone can seemingly agree upon one thing, best summed up by Mayweather Promotions CEO Leonard Ellerbe, that this was a sparring session in name only. 

"With Errol, that shit was a fight," Ellerbe recalled during a media scrum before Spence's 2019 victory over Mikey Garcia. "It was a fight."

Sparring inside Mayweather's gym has long held a controversial reputation, bolstered when Showtime's "All Access" cameras chronicled a particularlt brutal session in 2014 ahead of Mayweather's rematch with Marcos Maidana. There is no clock or set number of rounds inside the ring Mayweather calls the "Dog House," just two boxers trading punches in headgear until one of them quits while the other members of the gym surround the ring and raise the intensity by repeatedly pounding on the apron.  

Even though Mayweather is more well known inside the ring as a calculated ring technician and defensive wizard, his reputation in the gym has been as someone who isn't afraid to physically dominate his sparring partners, a belief that was corroborated by former lightweight champion Mickey Bey, who was promoted by Mayweather and was present for the Spence sparring.

"Floyd is used to beating the hell out of people," Bey told Trill Boxing Talk in 2022. "By the time camp is over, usually Floyd damn near ruined a fighter's career. I've seen him ruin a few young fighter's careers because Floyd is far different than when he fights. When he spars, he's trying to knock you out."

The difference between Spence and other Mayweather sparring partners ultimately came down to more than just natural ability. Spence told Showtime in 2017 that he didn't enter the ring that day to spar as much as, "I went out there like he was my opponent." And when the pounding of the mats from those crowding around the ring began to echo around the gym like the daunting beat of a drum, Spence said his competitive instincts shifted into overdrive. 

"I was like, 'Oh, it's on now,'" Spence recently told Champside

The single round of intense sparring reached upwards of seven minutes until Mayweather, who had buckled Spence with a right uppercut, according to cornerman Nate Jones, decided he had seen enough and began to exit the ring. 

Mayweather, according to Spence, looked back and told him, "You are alright for a bitch," to which Spence, by all accounts, immediately took issue and refused to leave. 

"I said, 'Man, you got me messed up [with someone else]. Put your shit back on,'" Spence recalled. "I said, 'Matter of fact, turn off the clock. We can go until somebody drops.' Everybody in the gym was like, 'Man, that's just Floyd, man, chill. That's just how he talks.' I was like, 'That's how he talks to y'all. He doesn't have to talk to me like that.'" 

The two-way action that followed lasted nearly 10 minutes, without a break, until Mayweather declared it was over. But not without Spence gaining his respect, which has continued to this day, even after Spence declined Mayweather's attempts to sign him to his promotion.

"Usually people fold under pressure and Floyd will cut that clock off," Bey said. "He cut the shit off and said, 'There ain't no bell, we are going to f---ing spar until somebody drops.' But Errol, his ass stood in there and was throwing leather with him. Errol told him, 'I'm not going nowhere.' He gave Floyd the best work I've ever seen."

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While the feisty character shown by Spence in his close encounter with Mayweather left little doubt in the minds of those who were present that Spence would one day become a champion, it's the evolution of his complete game 10 years later that has him on the verge of becoming an all-time great. 

Along with unifying a trio of 147-pound titles and making six defenses, Spence has built the deepest resume of anyone in the division over that stretch thanks to victories over Kell Brook, Lamont Peterson, Mikey Garcia, Shawn Porter, Danny Garcia and Yordenis Ugas. But it was a 2016 victory over former 140-pound champion Chris Algieri in Spence's first legitimate test on the elite level that truly announced his arrival less than four years after turning pro. 

Algieri, who once brought a 20-0 record into a 2014 PPV bout against Pacquiao, assumed he would be able to use guile and movement to frustrate Spence and expose his lack of big-fight experience. Just over four rounds later, Algieri tasted the canvas three times in brutal TKO defeat that showcased just how mature the young Spence's game had already become. 

"I underestimated him, for sure. Up to that point, we hadn't seen how great he can be. The potential wasn't that obvious," Algieri told CBS Sports on Monday. "Then, in the first round, I hit him with a right hand to his chest and I felt like I punched a pillar. It was like a cement wall. I was like, 'Oh man, I'm really going to need my legs, this is going to be a long night.'"

While Spence relied on his next-level confidence and competitiveness to leave his mark (and bolster his growing reputation) against Mayweather, by the time his 20th pro fight against Algieri had rolled around, he had figured out how to combine his sublime offensive skill with an NFL-ready muscular frame that he quite literally uses as a weapon. 

Algieri, who has made a successful transition post-retirement as a television analyst for multiple networks, compares Spence physically to "The Terminator" and said he believes his former opponent is the biggest and most dangerous welterweight boxing has seen since Hall of Famer Thomas Hearns.

"[Spence] makes the ring small. He's 5-foot-10 with a 72-inch reach and he's physically strong with wide shoulders," Algieri said. "You have this impressive and physical guy who is also one of the busiest champions in the world in terms of punch output. You have a guy who is big for the weight class, cuts a ton of weight but also his endurance is through the roof. 

"He's one of the most consistent body punchers in the game and that puts a lot of mental pressure on you. With Spence, you also have that throwback mentality where it's, 'This is a fight and I'm going to hurt you when I hit you. You may hit me with your best but I'm going to get it back and I'm going to get it back tenfold.'" 

Spence isn't just physically imposing, he's incredibly durable and has never been down as a pro. He has even survived detached retina surgery, which kept him out for the entirety of 2021, and miraculously walked away from a scary 2019 car crash that saw him ejected from the car at a high speed, without missing a beat inside the ring each time he returns. 

Even though a 2022 title unification win over Ugas in Spence's last fight was the most devastating example of the Texas native's weaponized physicality as he stalked and bullied Ugas before breaking his orbital bone to force a stoppage, Algieri has come away most impressed throughout Spence's evolution with his craft and technique.

Algieri's point of reference for Spence's world-class boxing skills from distance was a 2019 PPV win over unbeaten Mikey Garcia. After hearing pre-fight hype regarding how the pound-for-pound ranked Garcia was the better pure boxer of the two, Spence made it a point to use his length and footwork to silence any whisper of offense from his opponent in a 12-round masterclass that showcased Spence's cutthroat level of competitive fuel.

"I actually think [Spence] is underrated. He's a much better boxer than he gets credit for and I noticed that when I was in the ring with him," Algieri said. "He was more difficult to hit than I anticipated. His in and out was really good, which I hadn't really seen. But if you don't get his respect early, he's going to walk you down and beat you up all night long."

Spence proved against Brook and Porter, the two opponents he shares in common with Crawford, that he's just as capable of biting down and outdogging his foes as he is cerebrally solving them. Spence's ability to adapt and be exactly what the situation requires, while doing so in such a destructive and suffocating manner, is what Ellerbe believes makes Spence most dangerous. 

"He's not the fastest guy in the world, he's not the strongest guy in the world. But one thing about Errol is that he's very consistent and it's hard to beat a guy who is young, athletic and very consistent," Ellerbe said. "He gets into a rhythm and it's hard to beat a big, strong guy with a good, educated jab who understands the game and is in great shape for 12 rounds and is looking to punish you. More importantly, what I found out first hand, is you need to have some shit to keep him off of you. 

"I watched [Spence] and Floyd in camp. You have to think when you fight him. He puts mental pressure on you that forces you to make a lot of mistakes because he's not going to go overboard with certain things. Errol hits you to the arms and the body and you hurt. It ain't always about hitting you to the side of the head. Errol breaks you down over time."

Should Spence achieve his goal of becoming an undisputed champion, he would take the baton from Mayweather as the next great welterweight superstar. Yet,10 years later, people are still asking Spence about the existential meaning of him fighting on even terms in a mythical sparring match against the greatest boxer of this century. 

The more we get to know about what truly went on that 2013 day, the easier it becomes to understand how Spence got here, just days out from the most important fight of his increasingly legendary career, and how that same ruthless spirit of competition that was apparent against Mayweather is still the fuel that has made the unbeaten success of Spence's journey possible.

"It showed how gritty I am and how motivated I am," Spence told CBS Sports HQ on Wednesday. "When I came into that gym, they thought they can intimidate you by hitting on the ring mats and everyone yelling and screaming at you. I was kind of nervous at first but what they did, they motivated me. It really took the nerves away with people thinking how I would easily get beat up and not know how to handle myself. And I handled myself way better than people expected."

Who wins Errol Spence Jr. vs. Terence Crawford, and which prop is a must-back? Visit SportsLine now to see Peter Kahn's best bets for Saturday, all from the boxing specialist who has netted his followers a profit of nearly $4,000, and find out.