Their styles have been analyzed endlessly and their minds dissected as much as possible. And it still remains anyone's guess just what kind of fight this will be.
Will Pacquiao score early and often to beat a fighter who has never been beaten? Will Mayweather risk standing and trading punches, or be content to use his defensive wizardry to win for the 48th straight time?
Will boxing get the fight the hype deserves, and the sport desperately needs?
The fight is for the welterweight championship of the world, but in reality it's much more. Mayweather will be risking his legacy against arguably the best opponent of his career, while Pacquiao will carry the weight of an entire nation into the ring at the MGM Grand arena.
"Everyone talks about the money, the money, the money," Mayweather said. "I want the fight to live up to its magnitude. That's what it's really about."
Whether Mayweather actually believes that or is trying to sell pay-per-views — at a record price of $99.95 — won't be known until after the two men enter the ring sometime after 8 p.m. PDT. In past fights he's worried more about protecting his unblemished record than he has pleasing the fans who pay to watch him fight.
But he has clearly bulked up for this fight, returning to some old ways by chopping up tree trunks to gain muscle. He's going to be the bigger fighter in the ring, and he's going to have a chance to impose his will on Pacquiao if needed.
"Floyd Mayweather is going to try and take Manny Pacquiao's head off," said Mayweather's promoter, Leonard Ellerbe. "You can count on that happening. He'll do it in a manner where he follows a game plan, but Floyd Mayweather is looking to finish him off and I strongly believe that will happen."
Oddsmakers don't believe, making it a 3-1 favourite that the fight will go at least until late in the 12th round. The argument can be made that Mayweather hasn't legitimately knocked out an opponent since he stopped Ricky Hatton in 2007, and feels no pressure to do so with Pacquiao despite a payday that will likely be at least $180 million.
"I truly believe I'm the smarter fighter," Mayweather said. "He would be a better fighter if he wasn't so reckless. It's a gift and it's a curse. He's won a lot of fights by being reckless, but you can be reckless and get knocked out. And getting knocked out in a harsh way can affect you in the long run."
Pacquiao, of course, has been knocked out, in the harshest of ways. He was on the attack in the sixth round of his December 2012 fight with Juan Manuel Marquez when a right hand from Marquez left him face down and unconscious on the canvas.
To beat Mayweather, though, Pacquiao will have to risk getting hit. More importantly, he'll have to figure out how to get to Mayweather without taking big damage in return.
"He is a little bit bigger but like David and Goliath, size doesn't matter," Pacquiao's trainer, Freddie Roach said. "We are the better puncher and we are faster and have a much better resume."
For the first time in his career, Pacquiao studied video — and a lot of it — of an opponent. He and Roach spent hours trying to figure out how Mayweather sets traps for his opponent, then lures them in to get hit.
They're prepared for that, but they're also prepared if Mayweather becomes a different fighter in the biggest night of his career.
"I get the feeling they might start fast because he's put a lot of muscle on," Roach said. "I think he might try and catch us cold, but that won't happen. But I think he will come out and try to fight us."
If that happens, it might be because the fight took so long to make. Neither fighter has the same speed or reflexes they had five years ago, which some believe may play into Pacquiao's hands because Mayweather doesn't have the legs at the age of 38 to move around the ring for 12 rounds.
Assuming that proves true, there won't be many people complaining about paying $10,000 for ringside seats or the price of a pay-per-view at home.
"I think for pure boxing people it's going to be a fantastic, amazing boxing match," former heavyweight champion George Foreman said. "I think it's one that will live up to all the hype. It really will."
The fighters themselves tend to agree.
"I've never wanted to win a fight this bad in my life," Mayweather said.
"I know I'm going to win the fight in the ring," Pacquiao told a crowd of supporters this week. "So relax."Suggest a correction