This was by far the most dramatic of their contests over eight years and four weights, and the promoter Bob Arum, celebrating his birthday with surely as big a smile as in any of his 81 years, said: "A fifth fight? Why not? Have you seen a more exciting fight in years?"
The answer had to be universally in the negative to the last question.
Yet the knockout finish – to a fight with no title but of worldwide interest - instantly inspired more controversy, coming as it did after speculation by Pacquaio's trainer Freddie Roach in recent weeks that the Mexican's rippling muscles might not have been all of his own making.
And there will be others who wonder where the power arrived from. Rarely in 61 fights over nearly two decades can the 39-year-old Mexican have hit so hard in one contest (he had Pacquiao down in the third with a sledgehammer right that caught him with all the suddenness of a runaway truck).
While some social media doubters sailed close to libel in their post-fight claims of chemical assistance - a charge Marquez had vehemently denied since their third fight, in November last year - it has to be said, that nobody has ever done that to Pacquiao.
Apart from a couple of early-career stoppage losses in his homeland, one from a body shot, he has looked beyond destruction. If the Congressman decides to spend more time with his constituents at home, few would blame him. As the little man lay curled in a beaten heap, his wife, Jinkee, rushed from her seat, clambering through the ropes to lift his battered head. A year ago, their marriage had been in some difficulty. Last night, they could not have been closer. She might not want to see this again.
But there were already murmurs that Pacquiao is seeking a rematch. On the evidence of this candidate for fight of the year, or even the decade, it would not be hard to sell.
Pacquiao had Marquez down, as well, courtesy of a crisp, short right to the chin in round five. It marked the high-point of a breath-taking exchange of wild swings, as they abandoned all pretence at boxing. The judges, for once, all saw the fight the same way, and had Pacquiao ahead 47-46 after five rounds, agreeing, too, on each of the rounds.
The ending, however, shocked every one of the 20,000 customers who'd packed the Grand Arena at the MGM. This was high-quality boxing drama in an era crying out for excitement.
"I knew I could be knocked out at any time," Marquez said later, "when he was coming at me in those last three rounds, but I threw the perfect punch."
Pacquaio, once he'd recovered, responded: "I was able to come back from the first knockdown but the second one was a really good shot."
This was the fourth instalment of a series which had previously seen Pacquiao twice victorious, plus a draw in the first contest.
Marquez constructed his familiar shield of feints and beautifully timed counters from the start and Pacquiao bounced in front of him, flashing gloves at the ready. After the first three minutes, there was nothing in it.
Pacquaio, emboldened by a couple of long rights that pierced the older man's guard at the start of the second, steadied the Mexican in centre ring with a body shot that drew an intake of breath from his opponent. The Filipino did just enough to edge the round.
Pacquiao moved with greater urgency than he did a year ago, when he was awarded a decision against all popular and rational sentiment. But Marquez was sharp, too – and then came the thunderous bombshell: a right hand around a raised glove that rattled Pacquiao's teeth and felled him heavily, dazed and hurt. He shook his head and finished in a blaze of leather, as the roar hit deafening proportions.
Pacquaio said he wanted a war – and he got one. Only a knockout would do, he said; although the harder he looked for it, the tougher it was to find. In the fourth, he fought with anger and frustration but could not claw back the initiative as Marquez stood his ground. The Mexican looked strong.
The second knockdown of the fight came as suddenly as the first – and this time it was Marquez who went south, courtesy of a crisp, short right to the chin in round five. It was reckless, thrilling, and, with all notions of the boxing art temporarily shelved, nothing more or less than a bar fight.
This was don't-look-away stuff, a fight that had even seasoned fight writers rising from their seats. Marquez, bleeding freely from the nose and mouth, had not fought with such intensity in recent memory and, when he delivered the final punch of the evening, there was that familiar silence before the tumult.
Marquez is a remarkable fighter in his own right, although he will always be judged alongside Pacquiao. Now he has earned dominance with a single punch. A five-time world champion at four weights after being virtually ignored in the first half of his career, he remains the consummate ring master, his every move calculated to cause maximum irritation to his opponent. He always had Pacquaio's respect – now, even more so.
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