In the end, Pacquiao's third-round knockdown of Marquez provided the one-point difference on the judges' scorecards, enabling the pride of the Phillipines to escape with a 12-round split decision and dethrone Marquez for the WBC super featherweight championship.
Both fighters sustained nasty cuts that clouded their vision over the latter rounds of the hellacious fight, seen by a capacity crowd at Mandalay Bay Resort & Casino and a worldwide television audience on HBO Pay Per View.
The eventual outcome also was not so clear as the two fighters waged what resulted in almost a near repeat of their epic draw May 8, 2004.
But it was the lone knockdown, which gave Pacquiao a 10-8 round in the third, that proved the difference as judge Tom Miller scored the fight 114-113 for Pacquiao while Duane Ford scored it 115-112 for Pacquiao and Jerry Roth had it 115-112 for Marquez.
In their fight four years ago, Pacquiao knocked down Marquez three times in the first round only to spend the next 11 rounds trying to fend off his Mexican opponent's fierce comeback. The non-stop action resulted in each fighter getting the nod from two of the judges while the third judge called it a draw.
By contrast, the only knockdown Saturday night occurred when Pacquiao's ferocious left hook caught Marquez during an exchange. Marquez got up quickly and showed referee Kenny Bayless he was okay, but he nearly went down again from Pacquiao's right hand just before the end of the round.
From there, the fighters went back and forth hurting each other with wicked shots, heightening the drama in the seventh round when an accidental head butt opened a cut on the side of Marquez's right eye. Pacquiao also suffered a split right eyelid in the seventh round, and his cut at the time seemed more serious than Marquez's because of the location.
While Marquez succeeded in battering Pacquiao's eye, he also paid a price. The continuous ebb-and-flow action left Marquez with another deeper cut over his right eye. Pacquiao, admittedly, was bothered by the blood in his eye. But with the help of a masterful job from his cut man, Joe Thomas, he was able to weather the storm and survive the sea of red.
"It really bothered me because I couldn't see his right hands and hooks,'' said Pacquiao, who lifted his record to 46-3-2 (with 35 KOs). "I knew this fight was goignto be hard, so I prepared hard."
Of the knockdown blow, Pacquiao said "That was the No. 1 plan for training, the left hook." But he gave Marquez credit for taking him to the limit in an even more taxing fight than their first.
"He moved faster tonight with head movement and more counter punching,'' Pacquiao said. "I'm happy I won. He was a really hard opponent."
Marquez's record dropped to 48-4-1 (35 KOs) in becoming the latest Mexican champion to be beaten by Pacquiao, who owns two victories apiece against Marco Antonio Barrera and Erik Morales.
While Pacquiao, 29, is expected to move up from 130 pounds to challenge lightweight beltholder David Diaz, who won a non-title bout on Saturday's undercard, Marquez, 34, came away feeling he did enough to retain his super featherweight crown despite getting knocked down.
"We pressed him and we hurt him three or four times,'' Marquez said. "It's just like before, it's the whole fight not just one round."
Asked how the defeat will affect his future, Marquez was defiant. "I haven't lost anything at all,'' he said. "The popele will see. They know who really won the fight. We think we won."