The countdown is on to Saturday's showdown between Manny Pacquiao and Floyd Mayweather, the $400 million "fight of the century" with a place in boxing's pantheon of greats on the line.

More than five years in the making, it's an epic clash of styles and personalities, pitting the craftsmanship and defensive savvy of "Money" Mayweather against explosive southpaw Pacquiao -- an iconic figure in his native Philippines.

The welterweight world title unification bout looks set to smash boxing records for worldwide viewership and revenue, with Pacquiao promoter Bob Arum predicting it could generate as much as $400 million.

With a 60-40 purse split in favor of Mayweather, the unbeaten American stands to make an eye-watering $150 million and Pacquiao $100 million.

Fanned by instant Internet publicity and social media, global interest in the contest has skyrocketed.

The precious 500 tickets for seats at the 16,800-capacity MGM Grand Garden Arena in Las Vegas that were put on sale directly to the public at face values ranging from $1,500 to $10,000 sold out in minutes.

Promoters are even selling 10,000 tickets to Friday's weigh-in, at $10 a pop.

The audience on fight night will read like a who's who of A-list celebrities, casino high-rollers and others with the wherewithal to snaffle tickets that are going at secondary sales outlets such as StubHub for as much as $100,000.

The staggering financial figures and celebrity sideshows have only boosted the fight's cross-over appeal.

The build-up has focused on such minutiae as Mayweather's custom-made mouth guard, infused with diamonds and gold, and the $2 million-plus Pacquiao will rake in for advertising on his trunks.

Clash of characters

But once all that is stripped away, it will be up to the two men in the ring to deliver on the hype, in a duel that many feel has passed its sell-by date.

"The only thing the fighters can do is go out there and perform, and do what we do best," says Mayweather, who brings an impeccable 47-0 record with 26 knockouts to the bout, along with the World Boxing Council and World Boxing Association welterweight world titles.

Mayweather, trained by his father Floyd Sr. and continuing a family boxing legacy that stretches back for decades, is closing in on the iconic 49-0 record of 1950s legend Rocky Marciano, who retired as an undefeated heavyweight champion.

The American has held 11 titles in five weight divisions, his untarnished record and unabashed swagger making him the highest-paid sportsman in the world, according to Forbes.

Underlining his gift for provocative self-promotion, Mayweather last week compared himself favorably to heavyweight icon Muhammad Ali.

"I know there will be a backlash, but I couldn't care less," Mayweather said.

But beneath the glitzy surface runs a darker Mayweather story, studded by incidents of domestic violence.

It only makes Mayweather the perfect foil for Pacquiao, beloved in the Philippines as a humble humanitarian who has put his own womanizing ways behind him with a return to his Christian faith.

A lover of music and basketball, Pacquiao is a two-term congressman who many predict will one day be president of the Philippines.

At 38, Mayweather is two years older than World Boxing Organization champion Pacquiao, who is nevertheless considered the more ring-worn of the two, with his record of 57-5 with two drawn and 38 knockouts.

The only fighter to win eight world titles in as many weight divisions, Pacquiao's stock plummeted with two defeats in 2012, including a crushing one-punch knockout by Mexico's Juan Manuel Marquez. There were rumors of impending retirement.

He has since won three fights, although in his last bout, in November, he was unable to finish off Chris Algieri despite knocking him down six times.

Pacquiao hasn't knocked out an opponent since a TKO of Miguel Cotto in 2009 -- the same year he crushed Ricky Hatton in two rounds.

Mayweather's last knockout win was back in 2011 and he too has showed signs of ring-wear, though that has not stopped him going into the fight as the favorite.

Big, but how big?

Even so, the bout tantalizingly brings together two of the most talented fighters of their generation in a clash that recalls such past classics as Joe Frazier's victory over Ali in 1971, the first fight of their epic trilogy.

"The whole country stopped," the promoter Arum recalled of the immense interest in that fight at Madison Square Garden.

Whether Pacquiao-Mayweather will surpass Ali-Frazier -- or any other "fight of the century" -- in lasting signficance is immaterial, Arum said.

"This fight is tremendous," he said. "The interest is tremendous. And we should really wrap ourselves around that fact rather than compare it to fights of another era.

"One thing is clear: it's the biggest fight of this century.”