After months of hype and profanity Floyd Mayweather and Conor McGregor take their war of words into the ring on Saturday for a boxing mismatch that is poised to become the richest fight in history.

From motorcycle daredevil Evil Knievel to off-the-wall comedian Carrot Top, the Las Vegas strip has seen it all, but nothing quite like the crossover fight between undefeated boxer Mayweather and mixed martial arts champion McGregor.

Boxing purists have dismissed the bout as nothing more than a cash grab with McGregor in line for a $100 million pay day and Mayweather adding upwards of $200 million to push his career earnings towards the $1 billion mark.

Despite what is by most barometers a massive mismatch there is global fascination in the fight that could prove to be the richest in boxing history.

Promoters have sold the bout as "historical," a fight that will be talked about around the world for years to come and many intrigued fight fans have bought into the hype.

A crowd of close to 20,000 is expected to fill the T-Mobile Arena, where ringside seats on resale sites are being offered for more than $100,000, with millions more around the globe watching on pay-per-view.

There will be no titles on the line, only a gaudy "Money Belt" with a strap made from crocodile leather and featuring 24 karat gold, 3,360 diamonds along with sapphires and emeralds, but much will be riding on the result.

For Mayweather it will be the culmination of a life's work, a chance to surpass former heavyweight great Rocky Marciano, who retired with a perfect 49-0 career record.

For McGregor, a victory would give him the greatest upset in combat sports history and establish him as a crossover star able to choose big money fights in both the octagon and the ring.

The brash, charismatic Irishman, also known as 'Mystic Mac' for accurately predicting the outcome of his fights, has gazed into his crystal ball and says he will knock out the 40-year-old American inside two rounds of the scheduled 12-round bout.

Mayweather, a 12-times world champion over five weight divisions who traditionally fights defensive battles, is also calling for a knockout, something he has not done since stopping Victor Ortiz nearly six years ago.

"I’ve said it’s not going the distance and you can mark my words," said Mayweather. "I know that I’m in for a tough fight but there is one thing I do know, this fight is not going the distance."

McGregor, who was on social welfare from the Irish government as recently as four years ago, is the blue collar populist champion but the bulk of the money being wagered is all in Mayweather's corner.

Smaller bets are going on McGregor with UFC fans backing up their loyalty with their wallets. But the Las Vegas smart money is all streaming towards Mayweather with reports of several $1 million bets being laid on the 'Money Man.'

So sure is Irish bookmaker Paddy Power of the outcome that they have already paid out on a Mayweather victory.

As far as McGregor is concerned, anyone betting against him should take their money onto the Strip and set it on fire.

"I will go forward and put the pressure on and break this old man," said McGregor. "I don't see him lasting two rounds, I feel I will have the decision to end it inside one.

"He’s a beaten man. He will not be able to take the ferociousness that I come with."

Aside from McGregor's cocky assurances there is little in the tale of the tape to tip an upset.

Mayweather, who will bring a 49-0 record into the ring, is already recognised as one of the greatest fighters of all-time, a brilliant defensive tactician and counter puncher who has ruled the ring for 21 years.

In the other corner stands hard-hitting McGregor, a two-time world champion in the octagon making his professional boxing debut who will try to do what a long list of great fighters have been unable to do -- stop Mayweather.

McGregor's potential to register the biggest upset in boxing history rests largely with the intangibles.

As aggressive in the ring as he is outside it, the 29-year-old Irishman will have youth and power on his side.

A fearless and vicious fighter taking on an ageing boxer who hasn't fought in almost two years, McGregor's best shot at a victory will be connecting with a thundering left hand like the one that dropped Jose Aldo 13 seconds into their UFC featherweight championship fight.

"Everyone knows I can fight. I can give it and I can take," said Mayweather. "After 21 years I’ve been hit with everything and I’m still right here."