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 up and 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 welterweight fighting beyond 147lb for the first time, 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.
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.
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 he barely left a mark, beyond scoring. When Alvarez did there was a fear the Briton could instantly be hurt.
At times, Khan made Alvarez 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 on his chin and sent him flying to the canvas so heavily it was obvious the fight was over. He eventually returned to his feet and the champion could celebrate making the first successful defence of his title.
Afterwards, Khan said: “I’m a natural 147lbfighter. But this challenge came and it was very hard to turn down. I want to be the best. I want to fight the best and that’s why I took this fight. I showed my bravery by getting in the ring with Canelo. I wanted to go out there as a champion. I was unfortunate that I didn’t make it to the end, but I tried my best. I will probably go down to 147 now.”
Alvarez, who knelt beside Khan to check he was okay after delivering the knockout blow, stressed he was prepared to next fight the WBA and IBF champion Gennady Golovkin – who was present at the T-Mobile Arena – in a unification title fight. He said: “It was a human reaction to make sure he was okay and I’m glad he appears to be,” the 25-year-old Mexican said.
Earlier, there was victory for Ireland’s middleweight Jason Quigley, who earned a unanimous decision over another Mexican, James De la Rosa.