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".
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'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. A right hand staggered him in round one, allowing Crawford to send him to the canvas.
He was unable to live with the champion's slickness 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."
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.
'Did you quit? Tell the truth'
British rival Kell Brook said he thought Khan "quit".
In the post-fight news conference Khan was responding to claims he had done so when Crawford interrupted by asking: "Did you quit? Tell the truth".
Malignaggi added: "It was on its way to being a stoppage. That's probably the best way for Amir to leave the ring because it means he's not going to take any more punishment. He wasn't going to get back into the fight.
"Khan just needed a moment to be done. That was his moment."
Khan's trainer Virgil Hunter 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."
Radio 5 Live analyst Steve Bunce said Khan had been struggling with an elbow injury, adding: "All he kept saying in the ring was how sorry he was for letting people down.
"I've seen fighters in small halls getting thrown out for shots like that, accidental or intentional."
Khan - a former two-weight world champion - has faced long-standing criticism over his durability but saw this as a chance to again join the sport's elite by humbling a man often lauded as the best fighter across any weight division.
Defeat was always likely but the nature of the loss, along with pre-fight comments in which Khan said he was in the final chapter of his career, will pose questions as to what he does next.
A meeting with rival Brook would undoubtedly sell - although perhaps not with as much fever as when they were at their peak - and that appears the most lucrative contest left.
Khan, who shot to prominence when he won a silver medal at the 2004 Olympics, has stepped into the ring with some of the sport's stellar names and if his career were to end, Bunce believes he would be "in the top 25 British fighters of all time".
Malignaggi added: "He may get criticism but I can't fault him because he's had so many tough fights and he deserves a break."
Crawford, meanwhile, continues to offer a dazzling blend of poise, counter-punching and ruthlessness, positioning him for further greatness.
He has held all four belts at super-lightweight and has now won all of his 35 fights.
He dictated against Khan, switching from southpaw to orthodox stance at will.
"I saw a different Crawford tonight," added Bunce. "He was so comfortable at any distance and he can fight with any stance he wants to. I was greatly impressed."