Amir Khan insisted he could not go on after a low blow brought his WBO welterweight title challenge against Terence Crawford to a crashing halt.
The fight was stopped in the sixth round at Madison Square Garden after the technical knockout, handing the American the right to retain the belt and raising a cacophony of boos for Khan.
Speaking to ESPN in the ring as jeers rained down, Khan said: "First of all I want to apologise to all of the fans. The fight was just getting interesting.
"Terence is a great fighter, you know I'm not taking anything away from him. I now realise why he's one of the best pound-for-pound fighters in the world.
"I'm a good boxer but he was showcasing great skill, great movement. I was caught with a shot right below the belt and I could feel it. I was in pain."
Khan said he could feel the blow in his stomach and his legs "kind of seized" as a result.
He said: "I couldn't continue, I could not continue. I'm not one to give up any fight. I'll fight to the end, you have to knock me out to get me to give up you know. I was hit with a hard shot below the belt."
Crawford was unequivocal that his shot was above the belt and the decision to withdraw was not Khan's alone.
The Briton suffered a knockdown in the opening round and was taking regular punishment, struggling to stay in the contest and nursing what appeared to be an injury to his right hand.
He told ESPN in the ring: "First of all it wasn't a low blow. And second of all (Khan's trainer) Virgil (Hunter) knew the fight was going in a bad direction and saved his fighter before anything bad happened to him."
Hunter said safety was a key concern and Khan was "incapacitated" by the shot, but suggested the Briton was starting to get into the fight.
The trainer told BBC Radio 5 Live: "The crowd will always be bloodthirsty and want to see a dramatic ending but you have to look out for the safety of the fighter. He's not the kind of fighter to make things up. I believe he was incapacitated.
"We knew we were behind but Amir was starting to work things out and pick up his rhythm."
As for Khan's future, Hunter said his performance against Crawford was not "the type of beating that says he should retire".
"He showed he could move – his legs aren't gone – but he fought a hell of a champion tonight. I will be honest with him when I think it's time to retire," he said.