LAS VEGAS – The idea of Manny Pacquiao being knocked out cold was shocking.

The sight of him face down on the canvas, unresponsive as bedlam broke out all around him, was positively frightening.

Mitt Romney saw it up close from his ringside seat just a few feet away. So did Pacquiao's wife, who broke into tears and tried to get in the ring to aid her downed husband.

Juan Manuel Marquez didn't bother to look. He was already 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.

The fight will go down among the greatest of their era. But it was barely over when the cry arose for the two ever-so-willing warriors to do it again.

When it comes to Pacquiao and Marquez, four fights may not be enough.

"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 when Marquez landed the punch that sent him falling face first 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 their fourth meeting.

Pacquiao ended up paying the price for it when he tried to close the sixth round with a flurry, a big mistake against a counterpuncher 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.

One thing the stunning loss did do was scuttle, perhaps forever, what would have been the richest fight in boxing history.

With Pacquiao now damaged goods, any fight against Floyd Mayweather Jr. would be fought for a lot less money and generate a lot less interest than if it had happened with Pacquiao still on his winning streak and still in his prime.

But Pacquiao's career might not be over. If postfight comments from both fighters and promoter Bob Arum were any indication, Pacquiao and Marquez more than likely will fight for a fifth time. There's too much money to be had and the fighter in Pacquiao will surely want a chance at redemption.

That will be a hot topic of discussion in the months ahead. For now, though, one thing is for sure.

On this night, one huge right hand from Marquez changed everything.

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