Lennox Lewis retained the WBC title in controversial circumstances when Vitali Klitschko was stopped after the sixth round.
Klitschko had a deep cut above his left eye but led on the judges' scorecards when officials ruled he could not continue.
In the referee's opinion, the cut had been caused by a punch, which meant that Lewis was automatically declared the winner.
If it had been ruled that the cut had been caused by a headbutt, Klitschko would have won the title, as he was ahead on the scorecards.
The decision was greeted with boos around the Staples Center.
Lewis came into the fight heavier than at any time in his career, and it showed throughout.
The Briton looked in poor shape and was breathing heavily by round two as Klitschko landed several heavy right-hands.
A shock seemed on the cards, but Lewis gathered himself in the third and he received encouragement when a deep cut appeared above the challenger's left eye.
As early as the fourth round both fighters appeared close to exhaustion and each looked, at times, close to going down.
But Klitschko was dominant, frequently forcing Lewis back and stunning him with an array of punches.
The challenger proved his durability when two huge Lewis uppercuts in the sixth failed to drop him.
At the end of that round, Lewis slumped into his corner and looked every one of his 37 years.
But the Ukrainian had no chance to take advantage as the fight was stopped before he could come out for the seventh.
For a brief moment, it seemed like the fight would continue and it took some time for MC Michael Buffer to read out the official verdict.
Lewis was booed on his exit from the ring, while Klitschko's gallant performance was loudly cheered.
The champion said that a rematch was a possibility if "the money is right".
Lewis improves his record to 41 wins, two defeats and one draw, while Klitschko suffered his second defeat against 31 wins.