Calressa Shields has made history yet again. Just three years after becoming the first two-time Olympic gold medalist in U.S. amateur boxing history, Shields (9-0, 2 KOs) defeated Christina Hammer (24-1, 11 KOs) by unanimous decision to unify the women's middleweight division. All three judges were in agreement with 98-92 scorecards.

At times in boxing, it can be difficult to tell when two fighters truly dislike each other, or when they're simply drumming up publicity for the fight. In this matchup though, there was no question it was the latter. The multi-year lead-up to the unification bout was highlighted by in-ring altercations and sharp words everywhere from social media to interviews to press conferences. And when the final bell rang on Saturday night, the two fighters declined to embrace or even touch gloves. 

Shields, who has taken to calling herself the "G.W.O.A.T." (Greatest Woman of All Time) and reiterated as much after the bout over and over, put on a performance worthy of the moniker. After essentially giving up the first round, Shields completely dominated the fight. 

Though Hammer was known for her sharp jab, Shields showed off an impressive jab of her own. And even if she didn't actually land too many of them, Shields used her jab effectively to work her way inside, where she completely frustrated Hammer. The German champion wanted no part of fighting inside, and often resorted to grabbing and holding to simply get a break. 

Shields also displayed a much-improved defense, using slick movement to dip and dive out of the way of countless punches. Unable to land from the outside, and uninterested in mixing it up inside, Hammer simply had no answer. Still, Shields herself wasn't landing too many significant punches either. 

That all changed in the seventh round, and continued into the eighth, when Shields rocked Hammer with a combination that popped out Hammer's mouthpiece. The German pointed to the piece of equipment on the canvas, but the ref ignored it and Shields jumped back in to deliver some more punishment, before Hammer finally clinched to stop the action. Later in the round, Shields became relentless, pushing Hammer into the corner and unloading a flurry of punches that went unanswered. 

From there, Shields was on cruise control, and secured the historic victory without much trouble. 

With all four middleweight belts now hers, Shields wasted no time setting her sights on another undisputed champion: Cecilia Braekhus, who holds all the welterweight titles. 

In the ring after the fight, Shields said, "Women's boxing, we on fire! I cannot wait to see the next super fight, whoever it's between. And right now, I'm the undisputed champion. I want to fight Cecilia Braekhus at 154. Give me Cecilia Braekhus, the other undisputed champion. Me and her fight, that's pay-per-view."