Tyson Fury and Oleksandr Usyk will face off in a historic clash billed as the fight of a generation on Saturday with the first undisputed heavyweight bout in 25 years.

The mercurial Briton and the relentless Ukrainian have both arrived undefeated in the Saudi capital Riyadh, boxing's oil-funded new cash cow, looking to be crowned the sport's first four-belt champion.

Superlatives have been lavished on an encounter where one of the fighters can join the likes of Muhammad Ali and Mike Tyson by becoming the first undisputed heavyweight since Lennox Lewis beat Evander Holyfield in 1999.

With the hype going into overdrive, there is the potential for anticlimax at the Kingdom Arena with some commentators expecting a cagey fight with Fury on the defensive.

The 6ft 9ins (2.06m) Mancunian, who struggled against converted MMA fighter Francis Ngannou in October, has shed excess pounds, weighing in at 262lb, and looks leaner and light on his feet.

Southpaw Usyk, a converted cruiserweight with an astonishing resume, gives up six inches in height but has triumphed against bigger opponents, beating the towering Anthony Joshua twice. He has bulked up to 233.5lb.

Promoter Frank Warren called it the "most important fight of the 21st century".

"It's the fight we've been waiting for -- the two best heavyweights in the world, both undefeated," he said on a sweltering Thursday evening.

"This is something special. Fights like (this) come along once in a generation."

'Let's make history'

Opinions are split over the outcome, with some tipping the rangy, street-smart "Gypsy King" Fury and others backing the supreme skills and fitness of Usyk.

"Tyson Fury should win on points," Lewis told the BBC. "The bigger guy has longer arms, great movement."

However, Tony Bellew, Usyk's final victim at cruiserweight, warned: "He is the purest and best boxer Fury will ever face in his life."

"The guy is on another platform. There are boxers and then there is Usyk," Bellew added.

The final build-up has been explosive at times, including when Fury's father headbutted a member of Usyk's entourage and was seen with blood streaming down his face.

On Friday, his shirtless son shoved Usyk at the weigh-in, triggering a brief melee, and then hurled a series of angry F-bombs at the Ukrainian.

"I'm coming for his heart, that's what I'm coming for. (Forget) his belts. I'm coming for his heart, he's getting it tomorrow, spark out!" raged Fury.

Complaints about Saudi Arabian "sportswashing" -- using high-profile sport to deflect scrutiny of its human rights record -- have been virtually absent, with the promoters and fighters at pains to praise the conservative kingdom.

Fury and Usyk, who have a rematch clause, stand to profit handsomely from the fight, with reports claiming the Briton will bank at least £100 million ($127 million).

They are backed by noisy British and Ukrainian fans whose shouting matches have been ringing out across the swanky Riyadh Boulevard entertainment district.

Impeccable records

Fury holds the WBC belt, while Usyk took Joshua's IBF, WBA and WBO titles in 2021, winning their rematch in Jeddah a year later. Boxing has recognised four major belts since the 2000s.

Both fighters have impeccable records -- Fury is 34-0-1 with 24 knockouts, Usyk 21-0 with 14 KOs -- and engrossing back stories.

Fury, who comes from a line of bare-knuckle boxing Irish Travellers, has bipolar disorder and has battled alcoholism, cocaine abuse, depression and thoughts of suicide, announcing his retirement twice.

The 35-year-old famously upset another champion Ukrainian, Wladimir Klitschko, in 2015 and includes a memorable trilogy against America's Deontay Wilder in his portfolio.

Usyk, 37, who briefly served as a soldier after the Russian invasion, cleaned up as an amateur where he had a record of 335-15 and won an Olympic gold medal in 2012.

After then turning pro, he was the undisputed cruiserweight champion within 15 fights and claimed Joshua's belts in just his third outing at heavyweight.

Tales of his training are legendary, including 10-kilometre (six-mile) swims, four minute-plus breath-holds, juggling, and catching six coins at once to demonstrate his reflexes.

With little to choose between them, it may come down to whoever can stay smart and adapt over the 12 scheduled rounds.