LAS VEGAS -- Twice before, Manny Pacquiao and Juan Manuel Marquez waged spectacular action fights, both of which ended in controversial decisions.
There was the 2004 draw in a featherweight championship fight that featured Marquez climbing off the canvas three times in the first round to storm back, and Pacquiao's 2008 split-decision victory to claim the junior lightweight title.
Both fights were highly competitive. Since their rematch, Pacquiao has climbed through the lightweight, junior welterweight, welterweight and junior middleweight divisions en route to winning titles in a record eight weight classes, while Marquez -- a three-division champion -- eventually claimed the lightweight championship he holds today.
But in his one step up to welterweight, Marquez was schooled in a lopsided decision loss to Floyd Mayweather Jr. in 2009.
Now Pacquiao (53-3-2, 38 KOs) will defend his welterweight crown against Marquez (53-5-1, 39 KOs), who has bulked up again, on Saturday night at the MGM Grand Garden Arena in a much-anticipated third showdown.
But even considering the closeness of their first two fights, Pacquiao -- who has been fighting and dominating top welterweights since destroying the heavily favored Oscar De La Hoya and sending him into retirement in December 2008 -- is a heavy favorite in Chapter 3.
Last week, the stakes were raised when Mayweather, whose potential showdown with Pacquiao has been brewing for years and looms as boxing's biggest-money fight ever, said he planned to fight on May 5 at the MGM Grand, ideally against Pacquiao. But Pacquiao, obviously, has to win or the sport's biggest fight would go down the tubes.
But take a moment to contemplate this: What happens if Marquez pulls the upset?
Although the MGM sports book lists him as about a 9-1 underdog -- a huge long shot by boxing standards -- the Mexican star has given the Filipino icon the toughest fights of his recent career, other than Pacquiao's last defeat -- a close decision to Erik Morales in a 2005 junior lightweight bout.
Clearly, Marquez knows how to fight Pacquiao.
"That's why we are doing this third fight," Marquez said. "The first two were very close and this fight should end all doubt. We are not the only ones saying we won the fights. There are a lot of fans and media out there saying the same thing -- that we won those two fights."
Said Nacho Beristain, Marquez's Hall of Fame trainer: "Without question, we have prepared ourselves to win this fight again. They can say what they want. If they feel they won the first two fights, so be it -- we feel the same way and that's the way you should go into a fight."
Should Marquez win, the next step would seem pretty clear: Top Rank's Bob Arum, who promotes Pacquiao and signed Marquez for this fight, holds an option for an immediate rematch, which would theoretically take place next spring.
Arum guaranteed Marquez $5 million for Saturday night's fight (plus an upside of the pay-per-view profits). If Marquez wins and the option is exercised, Arum would owe him $10 million on the rematch and an even more generous slice of the PPV money.
Pacquiao and Marquez would be 1-1-1 against each other and would likely settle the score with a fourth fight, something rare in modern boxing.
In the event Marquez wins Saturday, "It would be a whole different world -- and not necessarily a bad world," Arum said. "In other words, I've been around this business so often that, they win, they lose and you play the hands that's dealt to you. And certainly if Manny loses, that's not something that's gonna make me happy because, personally, I'm so attached to him. But the fact that there's a rematch is a little bit of a balm."
Said Michael Koncz, Pacquiao's adviser: "I don't think it would hurt the landscape of boxing. We have a rematch clause. I don't have any fears or doubts, but there is no guarantee. Things happen, and if [Pacquiao loses], it's not going to hurt the sport of boxing."
Arum acknowledged, however, that the potential Pacquiao-Mayweather fight -- which Arum said he wants to make, even if some doubt him -- would be ruined, at least for now, if Pacquiao lost.
"Really, truly, I haven't given any thought to Manny's next step assuming he wins the fight," Arum said. "Obviously, we'll see if conversations -- adult conversations -- can take place with the Mayweather side if Manny wins. And if they can't take place, then there are other opponents we'll look at."
But if Pacquiao loses?
"It certainly would not be on the immediate radar," Arum said. "You play the hand that's dealt to you. One thing I have no control over is what happens inside that ring."
Pacquiao said he isn't thinking about Mayweather, only about beating Marquez convincingly and making sure there is no need for a fourth fight.
"I remember when I was 24 or 25 and how I trained hard, and this training camp I felt like that," Pacquiao said. "I'm very inspired to train hard and do sacrifices in training, and I started early for this training camp. It's because of what I need to prove. So that's why I train hard.
"Marquez is a tough opponent. I can say Marquez is not an easy opponent."
Freddie Roach, Pacquiao's trainer, has been predicting that Pacquiao -- because he is extra-motivated to make a statement by knocking out a fighter that Mayweather couldn't -- will stop Marquez inside six rounds.
But Roach has thought about what it would be like if Pacquiao lost.
"That would change a lot of things," he said. "All of a sudden, Mayweather would be the guy. It would be devastating for boxing because Manny's a hero right now. He's global, not just Filipino anymore. He would lose a big audience."
Arum did not agree with that assessment. He said because of how Pacquiao fights, he would retain his popularity.
"When he lost to Morales, that did not diminish his popularity, but that was when he was a much younger fighter and he a had a much bigger career ahead of him," Arum said. "Whether it will affect his popularity if he loses, I'd rather think not -- particularly if, knowing Manny, he will go out on his shield."
But Roach made the point that when Pacquiao lost to Morales, he was nowhere near the global phenomenon he has become.
"He wasn't quite at the level he is now," Roach said. "I think a loss would be devastating."
If Pacquiao-Marquez III is another entertaining fight, as their first two bouts were, Pacquiao and Marquez can embellish their reputations, said Mark Taffet, who oversees HBO PPV and has been involved in virtually all of their big fights.
"Whenever two great fighters get in the ring and give their all, there are no losers, regardless of the outcome of the fight," Taffet said. "If the fans get the performance that they hope for, their interest in those fighters never wanes. And there are always big fights down the road.
"Both Pacquiao and Marquez are heroes. And they carry their countries on their backs whenever they get into the ring. Marquez has had great fight after great fight and many, many big fights. As he is proving again as he gets ready to get in the ring with Manny Pacquiao, when he gets in the right fights and the right matchups, they are megafights and there is tremendous media and fan anticipation. With a country behind him, and all the fans he has, if Marquez is successful against Pacquiao there will be tremendous fan interest in his next fight."
If Pacquiao happens to lose, though, Roach said that his fighter might not need a fourth fight with Marquez after all.
"You get more offers off losses than a win sometimes," Roach said.
Then he cracked a smile, laughed and said, "Maybe then Mayweather will fight us."
Dan Rafael is the boxing writer for ESPN.com. Follow him on Twitter @danrafaelespn.