Juan Manuel Marquez has claimed victory over Manny Pacquiao after a stunning knockout counterpunch in the sixth round of their non-title welterweight fight in Las Vegas.
A bloodied Marquez landed flush with his right hand from the corner and sent Pacquiao face first to the canvas with just one second of the round remaining.
"I felt he was coming to knock me out the last three rounds, and I knew he was going to be wide open," said Marquez after the fight.
The result will likely put paid to any planned superfight between Pacquiao and Floyd Mayweather, with real questions posed over where the eight-division former world champion's career goes next.
It marks the second defeat in a row for Pacquiao, following his disputed title loss to Tim Bradley earlier this year.
An aggressive-looking Pacquiao had been ahead 47-46 on all three scorecards, and metrics showed he had thrown significantly more shots going into the sixth, which the Filipino looked to be in control of.
He had already been dropped once by the 39-year-old Marquez, with a strong punch in the third round. Pacquiao had recovered quickly from that to take control of the fight again, leaving the Mexican on wobbly legs at the end of the fifth after knocking him down.
Both men were exchanging heavy hands, but as the sixth round drew to an end Marquez' counterpunch connected with brutal force to give the Mexican a victory over Pacquiao at the fourth attempt. The Filipino was down for around two minutes before his team were able to get him up
"I got hit by a punch I didn't see," Pacquiao admitted,while trainer Freddie Roach said the fighter had been 'careless' from his leading position.
Afterwards Pacquiao indicated he would like a fifth fight with Marquez, whom he had controversially beaten twice going into the fight, which promoter Bob Arum confirmed could be on the cards.
But the often-mentioned $100m superfight with Mayweather now looks to be dead in the water, following Mayweather's jail spell and Pacquiao's latest defeat.