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 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 down in tears and tried to get in the ring to aid her downed husband.

Juan Manuel Marquez didn’t even 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 in the rafters of the MGM Grand arena.

It will go down among the great fights 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 himself when Marquez landed the punch that sent him falling face first on 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, then went to his hotel suite, where he ate with wife Jinkee and his entourage and watched a replay of the fight to see what went wrong.

“Spoiler alert,” Pacquiao said as the fight played on the TV. “I don’t think you are going to like how this ends.”

His countrymen in the Philippines certainly didn’t. The country came to a standstill as it usually does when its hero fights, and for the second fight in a row they were bitterly disappointed.

In the southern region where the boxer and congressman lives, some survivors of a powerful typhoon that killed more than 600 people this week watched on a big TV screen in a gym that serves as an emergency shelter in the town of New Bataan.

“People were really dismayed,” town spokesman Marlon Esperanza said. “It was like they were hit by another typhoon.”

What Marquez hit Pacquiao with might have seemed almost as powerful. 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 at 125 pounds 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.

The only question was which fighter would end the night on the canvas.

It turned out to be Pacquiao, who lost a controversial decision in his last fight to Timothy Bradley and who many in boxing believe is showing the wear of 17 years in the ring. For any other fighter the knockout loss might be the end, but Pacquiao showed no sign afterward that he was willing to call it quits on his remarkable career and return to his other job as a congressman in the Philippines.

Trainer Freddie Roach said the decision won’t be an easy one.

“I said if he is back in the gym and I see signs of him declining I’ll tell him to retire, but if I don’t see that I won’t tell him to retire,” Roach said. “I’d love to get a rematch, but is that the best move right away? Should we try him out in a softer fight first? There is a lot of things we have to think about. It’s very complicated, and it’s not going to be overnight.”

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.

Pacquiao’s career may not be over. If postfight comments from both fighters and promoter Bob Arum were any indication, he and Marquez will more than likely 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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