Juan Manuel Marquez delivered a devastating right hand to knock out Manny Pacquiao in the sixth round of their welterweight bout in Las Vegas.
Mexican Marquez, 39, floored Pacquiao, who lay motionless on the canvas for some time, for a first win over the Filipino in their four-match rivalry.
Pacquiao, 33, had won the previous two encounters, with one draw.
It was a second successive defeat for Pacquiao after a controversial loss to Timothy Bradley in June.
Pacquiao had already been down in the third round - the first time in 39 rounds between the two that the eight-weight world champion was knocked from his feet - as Marquez threw a right hook over the top.
But Pacquiao recovered to ground Marquez in the fifth and the pair were exchanging heavy blows in the sixth.
However, with one second of the round remaining, a bloodied Marquez threw a huge overhand right that had Pacquiao out cold before he hit the canvas under the ropes.
Pacquiao did not move for around two minutes before finally being dragged to his feet by his team.
Yet, he was well enough to be interviewed shortly after the fight and said he wanted to meet Marquez in the ring for a fifth time.
"Why not? It's a good fight," he said. "I did my best. I know that I got him in the last couple of rounds but I started to be over-confident and I got hit by a solid punch. I did not expect that punch."