Amir Khan's world-middleweight title dreams ended in the heartache of defeat in Las Vegas when WBC champion Saul Alvarez stopped him with a heavy knockout in the sixth round.
The 29-year-old had made the promising start to the fight he required by capitalising on his speed to build an early lead, but ultimately proved too small and not elusive enough to evade the bigger fighter for the 12-round distance when he was caught with a huge right hand.
The Briton stayed on the canvas for some time before getting to his feet and giving a lucid interview - but he was taken to hospital as a precaution afterwards.
Khan had always needed to use his range and superior speed and mobility to land scoring shots on the powerful Mexican, to excel early while at his freshest and when the champion was more patient.
The Briton, a natural welterweight fighting beyond 147lbs for the first time and clearly the smaller man in the ring, had spoken of his belief that he lacked the power to stop Alvarez - and his performance demonstrated that.
Khan sought to land and evade danger, beginning promisingly with jabs and fast combinations and taking what Alvarez threw back - until the elements of the unknown he had never experienced came into play.
The intense levels of concentration required to read his dangerous opponent - coupled with his need to continually move and the gradual influence of Alvarez's size and strength - could easily have drained the Bolton fighter's sharpness which was so key.
There was also the possibility that some of his speed would have been sacrificed by carrying the additional weight, and that not only would that also contribute to him tiring but that Alvarez would adapt and make the adjustments required to read and time his vulnerable challenger.
The fight threatened to turn when, in the third round and in the smaller ring that suited Alvarez's attempts to limit Khan's space, the 25-year-old champion began to find his range with the powerful left hook he repeatedly threw.
Khan retained his speed and continued to land in return but the difference was that when he did he barely left a mark, beyond scoring. When Alvarez did there was a fear the Briton could instantly be hurt, and that it would gradually catch up with him and slow him down.
At times Khan made Alvarez clearly miss and there were signs of the frustration the Mexican demonstrated when losing his undefeated record to Floyd Mayweather in 2013, particularly in the fifth.
Khan impressively took another left at the start of the sixth. Although hurt, he fought back to land further combinations.
However, almost from nowhere, a huge Alvarez right hook landed directly on his chin and sent him flying to the canvas so heavily it was obvious the fight was instantly over.
For a few moments there were fears Khan was seriously hurt as referee Kenny Bayliss waved the action over after two minutes and 37 seconds of the round.
However, he eventually returned to his feet, devastated, and the champion could celebrate making the first successful defence of his title.
Remarkably, only one of the three judges had Khan leading at the time of the stoppage, via a score of 48-47. The other two had Alvarez 48-47 and 49-46 in front.
The defeat and its nature does little to damage Khan's reputation or prospects. In receiving his biggest purse to date, he lost to a fighter few expected him to defeat and two divisions beyond his natural weight.
Earlier, there was victory for Ireland's promising middleweight Jason Quigley, who earned a comfortable unanimous decision over another Mexican, James De la Rosa, via three scores of 100-90. The win was Quigley's 11th from 11 professional fights.