Amir Khan's shot at WBO world welterweight champion Terence Crawford ended in bizarre fashion when a low blow saw him pulled out by his corner.
Khan, floored in the opening round, took an accidental shot to his groin in the sixth and after consulting his corner said he was in "too much" pain.
Crawford was well up on the cards but the nature of the stoppage led to brief concern he may have been disqualified.
"I have never been hit below the belt and was in pain," Khan said.
"First of all I want to apologise to all of the fans. The fight was just getting interesting."
Boos rang out at New York's Madison Square Garden upon the conclusion, prompting BBC Sport commentator Mike Costello to say Khan "could be in for a storm of abuse".
Former world middleweight champion Andy Lee said Khan had "done himself a mis-service", while former two-weight world champion Paulie Malignaggi said the Briton "wasn't going to get back into the fight".
'I could not continue'
Khan, a heavy underdog against an undefeated American who has held world titles in three weight divisions, looked nervy during his ring walk and a right hand saw him stagger in round one, allowing Crawford to send him to the canvas.
He was unable to live with the slickness the champion brought and took hard shots to the body in round four, landing sporadic - if light - punches of his own.
When Crawford drove a left hook into his groin in the sixth, Khan was legally allowed to take five minutes to recover but after around a minute, the bell sounded.
"I could feel it in my stomach and legs. I said 'I can't move'," Khan added. "There was no point taking five minutes out, I could not continue. I am not one to give up. I was hit by a hard shot below the belt.
"I couldn't continue as the pain was too much."
Boos were audible but Crawford appeared relaxed at the sound of the bell, despite general confusion as to whether he risked disqualification.
The champion led 49-45 50-44 49-45 on the scorecards at the time and immediately said he hoped to face IBF champion Errol Spence Jr next.
More to follow.
