Darren Barker lost his unbeaten record as WBC middleweight champion Sergio Martinez knocked him out in their bout in Atlantic City.
Barker, 29, suffered his first defeat in his 24th fight as the Argentine won in the 11th round.
The Briton was stunned with a right before the champion moved in with another right which landed behind his opponent's ear.
Barker went to the canvas and while he tried to get back up, was counted out.
Barker said: "I'm extremely disappointed and gutted not to be bringing the belt back home.
"I didn't think he was too much of a puncher but he caught me with something and my legs just went."
Victories for Martinez against Kelly Pavlik, Paul Williams and Sergiy Dzinziruk had seen him shoot up the pound-for-pound rankings to third spot behind Floyd Mayweather and Manny Pacquiao.
Barker drew blood from Martinez's nose in the fourth round with a good combination but the champion looked in control as he shook Barker with two rights in the 10th before finishing the job a round later.
Martinez paid tribute to his opponent, saying on Sky Sports: "I wasn't frustrated at all, it was a good job by Barker.
"I had to neutralise his attack, I had to break him down."