Baltimore's Gervonta Davis lost his belt by missing weight even before fighting Saturday and his inattention in the ring also led to a more difficult bout than he believed.
Referee Russell Mora controversially awarded Davis an eighth-round knockout victory on a left-handed punch that struck opponent Francisco Fonseca in the back of the head.
Davis (19-0, 18 knockouts) said he hurt Fonseca with a body blow before the punch, and explained that the disputed shot "capped" the damage, although fans who watched replays in the arena booed that thought.
Davis immediately swinging aggressive uppercuts to open the bout, trying to end the fight impressively, but he was locked in a tougher test than he thought.
Davis, after working out under Mayweather's guidance and training feverishly in a rubber suit Thursday night, revealed again the difficulty some boxers have in making weight.
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