Former Olympic light-heavyweight champion Andre Ward beat WBA super-middleweight champion Mikkel Kessler with a stoppage success in California.
The fight, part of the Super Six tournament, was ended in the 11th round because of a cut to the Dane's face.
It was a surprise defeat for Kessler (42-2), who was well fancied to win the super middleweight tournament.
"Right now, I'm dreaming," Ward said. "I can't compare this to winning the gold medal, because it's going to take a while for it all to sink in.
"Everyone said we got the bad draw, but we just beat the toughest guy."
Ward admitted there were two head butts which contributed to Kessler's injury, but said they were accidental.
"I'm not a dirty fighter, everybody knows that," he added.
However, Kessler was angered by his opponent's tactics, and insisted that he could have fought on.
"He was the better man tonight, but I would have liked the fight to go another minute," Kessler said. "I knew I could have continued. I was cut, but the cuts weren't that bad.
"He threw a lot of elbows, and there were a lot of head butts, and he never got any warnings. ... I don't want to sit here and complain all the time, but I wasn't happy with the referee."
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