Kazakhstan's Zhanat Zhakiyanov survived two first-round knockdowns to upset Rau'shee Warren by split decision on Friday and claim the WBA and IBO bantamweight world titles.
On the same card in Toledo, Ohio, unbeaten Robert Easter retained his International Boxing Federation lightweight title with a unanimous decision over Puerto Rico's Luis Cruz.
Warren, making his first defense of the titles he claimed with a majority decision win over Juan Carlos Payano, looked headed for a short night when he put the challenger down twice in the first three minutes.
But Zhakiyanov, trained by Britain's former two-division world champion Ricky Hatton, steadied and gradually seized control of the bout.
He hurt Warren with a pair of right hands in the third round and Warren was on the canvas moments later, although referee Gary Rosato ruled he'd been pushed rather than knocked down.
For the rest of the fight Zhakiyanov attacked relentlessly, his punishing body blows particularly effective against the American southpaw.
While one judge saw it 115-111 for Warren, Zhakiyanov got the nod from the other two by scores of 116-110 and 115-111.
Zhakiyanov, 33, was fighting for the first time since a split decision victory over Yonfrez Parejo in Monte Carlo in November of 2015. He improved to 27-1 with 18 knockouts while Warren fell to 14-2.
"We were surprised when Zhanat went down like that in the beginning of the fight. I just wanted to remind him what he does better and what Rau'Shee is good at. He is never going to outbox him; he had to get in his chest, he had to mug him and make it a brawl. Zhanat did that very well and that's how he won the fight," Hatton said.
"If we lost the fight, we would have been upset, but we'd know it wouldn't have been a bad decision. I know that Warren is gutted, he feels like he won the fight and let's be honest - it was the type of fight where no matter who won, the loser would have felt like he deserved to have his hand raised.
"Their styles make for a great fight. Why not do it again? Not just because it was close but because it was such a great action fight and it always will be."