RIYADH – 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 6'9" (2.06 meters) Mancunian, who struggled against converted MMA fighter Francis Ngannou in October, has shed excess pounds, weighing in at 262 pounds (lbs), and looks leaner and light on his feet.

Southpaw Usyk, a converted cruiserweight with an astonishing resume, gives up 6 inches in height but has triumphed against bigger opponents, beating the towering Anthony Joshua twice. He has bulked up to 233.5 lbs.

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."