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The more Kamaru Usman continues to stockpile title defenses on his march toward potentially eclipsing Georges St-Pierre as the greatest welterweight in UFC history, the number of viable opponents he hasn't yet beaten continues to dwindle. 

Usman defended his 170-pound crown for the fifth time at UFC 268 in his second straight rematch by edging Colby Covington via unanimous decision. At 34, "The Nigerian Nightmare" finds himself without peer at the moment atop the sport's pound-for-pound list and could make a run at the history books depending upon how much longer he continues to fight.  

Although Usman hasn't necessarily teased that a mic drop exit is imminent, recent abrupt retirements by former champions Khabib Nurmagomedov and Henry Cejudo suggest we are in the midst of a trend. And outside of a rising yet still raw Khamzat Chimaev, there isn't a slam dunk next choice for a fresh opponent considering Usman has already beaten Leon Edwards, and both Vicente Luque and Michael Chiesa fail to offer the champion a marketable reason to stay so active.  

Regardless of what happens next, Usman's incredible evolution over his last four fights under new coach Trevor Wittman has him on the verge of some rarified history. Usman is one win shy of tying Anderson Silva's promotional record of 16 straight victories. And even though he is still four title defenses away from equaling GSP's welterweight mark, the dominance behind his current win streak make it so Usman might not need to eclipse said mark to fully cement his legacy.

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Not only is Usman unbeaten in 15 trips to the Octagon, he hasn't be challenged or pushed by anyone not named Covington, whom he has now beaten twice in close fights. Usman has also won 19 consecutive fights overall since a loss in his second pro outing and enjoys a level of sustained dominance even St-Pierre couldn't match.  

Should Usman continue on in his winning ways, admission into the upper room of all-time great fighters with a case to be called the G.O.A.T. seems inevitable. Already a dominant wrestler, Usman has proved in 2021 with stoppage wins in title defenses over Gilbert Burns and Jorge Masvidal that he's now just as dangerous on his feet.  

And then there's the debate over his perfect takedown defense record, which stands alone atop the UFC record books. Some have argued that Covington came incredibly close to scoring one at UFC 268, yet it remains moot to the overall point.  

With Nurmagomedov's recent retirement and the combination of Jon Jones' decline and inactivity, Usman currently stands in a class of his own as the best male fighter in the sport -- and, somehow, he's only getting better.

For CBS Sports' updated divisional rankings, click here.

Men's pound-for-pound rankings

Dropped out: Brandon Moreno

Just missed: Moreno, Cyril Gane, Aljamain Sterling, TJ Dillashaw, Glover Teixeira 

Women's pound-for-pound rankings

Dropped out: None

Just missed: Jessica Andrade, Germaine de Randamie, Holly Holm, Marina Rodriguez, Katlyn Chookagian