Amir Khan's WBO world welterweight title fight with Terence Crawford ended in bizarre fashion when he was pulled out by his corner after a low blow.
Floored in the opening round, Khan took an accidental shot to his groin in the sixth and, after consulting his corner, said he was in "too much" pain.
American champion Crawford was dominant and later questioned whether Khan quit, urging the Briton to "tell the truth".
"I would never quit, I would rather get knocked out," Khan, 32, said.
"I have never been hit below the belt and was in pain.
"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 after the fight, and BBC Sport commentator Mike Costello said Khan "could be in for a storm of abuse".
Former world middleweight champion Andy Lee said Khan had "done himself a misservice", while former two-weight world champion Paulie Malignaggi said he "wasn't going to get back into the fight".
'I could not continue'
Khan - a heavy underdog against the undefeated Crawford, who has held world titles in three weight divisions - looked nervy during his ring walk and a right hand staggered him 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, 31, drove a left hook into his groin in the sixth, Khan was legally allowed to take five minutes to recover but, after about a minute, the bell sounded.
"I could feel it in my stomach and legs. I said 'I can't move'," Khan said. "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."
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When an accidental injury ends a bout in which four rounds have been contested, the judges' cards are used, but the announcement of a technical knockout meant Khan was stopped.
Crawford was leading 49-45 50-44 49-45 on the cards at the time of the stoppage.
There was brief confusion as to whether he might have been disqualified but upon being declared the victor he immediately said he hoped to face IBF champion Errol Spence Jr next.