LAS VEGAS - The idea of Manny Pacquiao being knocked out cold was shocking enough. The sight of him face down on the canvas, unresponsive even as bedlam broke out around him, was positively frightening.

Juan Manuel Marquez didn't even bother to look. He was busy celebrating the knockout of a lifetime.

This was boxing at its brutal best, a toe-to-toe slugfest Saturday night that was destined from the opening bell to be decided by fists instead of judges.

Both fighters had been down, and both fighters were hurting when Marquez threw a right hand off the ropes with a second left in the sixth round that could be felt all the way to the rafters of the MGM Grand arena.

It will go down among the great fights of their era. It was barely over when the cry arose for the two ever-so-willing warriors to do it again.

"If you give us a chance, we'll fight again," Pacquiao said. "I was just starting to feel confident, and then I got careless."

Indeed, the case could be made that Pacquiao was on the verge of a big win himself when Marquez landed the punch that sent him facefirst to the canvas. He had come back from a third-round knockdown to drop Marquez in the fifth and was landing big left hands that broke and bloodied the Mexican's nose.

After three fights that all went the distance, both fighters had vowed to be more aggressive in this meeting. Pacquiao paid the price when he tried to close the sixth round with a flurry, a mistake against a counter-puncher who drew him into his sights.

"I knew Manny could knock me out at any time," Marquez said. "I threw the perfect punch."

Pacquiao, who hadn't been stopped in a fight since 1999 in Thailand when he was a 112-pounder, took several minutes to come around on the canvas before being led to his ring stool. He blew his nose and stared vacantly ahead as the pro-Marquez crowd of 16,348 screamed in excitement.

He was taken to the hospital for a precautionary brain scan.

Pacquiao had dropped Marquez four times in their first three fights, but Marquez had never put him down before he landed a big right hand in the third round for his first knockdown. The power was sure to raise questions about the new bulked-up physique Marquez has at the age of 39, which he said came from hard work under a strength conditioner who once provided steroids to Marion Jones and other track stars.

Still, it was a career-defining moment for Marquez, who believes he was robbed by the judges in his first three fights with Pacquiao. The two fought to a draw eight years ago, and Pacquiao was awarded close decisions in the other two fights.

It was clear there would be no need for the judges on this night, which might have been good for Marquez, since he was losing by one point on all three scorecards when he landed his big punch.