Amir Khan's dream of finally proving himself one of the finest fighters in the world ended in a controversial defeat when he was stopped in the sixth round after a low blow by the great Terence Crawford at Madison Square Garden in New York.
Fighting to also become a two-weight world champion by adding the WBO welterweight title to his impressive list of honours, Khan had long pursued this opportunity which has likely ended his career at the highest level, leaving him with few remaining lucrative options.
A domestic fight with long-term rival Kell Brook will appeal until either retires, but for the 32-year-old Khan - groomed for greatness since winning an Olympic silver medal in 2004 aged 17 - his successful time as an attraction in the US may have passed.
He suffered a knockdown in the opening round and was thereafter taking regular punishment, struggling to stay in the contest and nursing what appeared to be an injury to his right hand.
When Crawford then landed that horrendous low blow he appeared to be taking advantage of the opportunity to take a lengthy recovery.
The fight was instead strangely stopped after 47 seconds of the sixth when his corner indicated he could no longer continue, despite him being entitled to far longer before fighting on.
Alongside Ukraine's Vasyl Lomachenko, Crawford has long been considered one of the world's two leading fighters, and he demonstrated why by largely easing in front until then.
Khan's finest chance came in him rediscovering the composure and discipline with which he excelled in once beating Devon Alexander and Luis Collazo before that devastating knockout by Saul 'Canelo' Alvarez, in the hope that his speed and volume would prove too much for Crawford to negate.
Instead the champion, 31, effortlessly used his ring IQ and sense of distance and timing to read Khan and essentially take control from that first-round knockdown.
Khan had made an encouraging start when he landed with his still-impressive speed, but he was hurt by a powerful right hand and then floored by a left hook, and thereafter largely left clinging on.
Instead of pursuing the immediate stoppage Crawford remained measured, and continued to instead pick his challenger off.
In the fourth he hurt Khan to both head and body, threatening to stop him on his feet, and while Khan typically fought back he lacked the same conviction and it was around then that he began to struggle with his right hand.
Further lefts and rights troubled Khan as he unexpectedly resisted another knockdown in the fifth but in the sixth - after he had already complained to his trainer Virgil Hunter about his hand - came the low blow that, while sickening, also represented an unsatisfactory conclusion and prompted boos.
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."
A fifth defeat - and the fourth inside the distance - demonstrated there is little question that Khan is a fighter in decline. Regardless of whether he fights on, it may be the nature of this stoppage and no longer that Floyd Mayweather and Manny Pacquiao eluded him when he was at his peak, that proves his biggest regret.
There were earlier convincing victories for promising Americans Shakur Stevenson and Teofimo Lopez, taking each closer to fighting for a world title. Stevenson earned a unanimous decision over 10 rounds against Christopher Diaz, and Lopez stopped Edis Tatli in five.