How long Tyson Fury or Oleksandr Usyk will really be allowed to call themselves the 'undisputed world champion'

The winner of Tyson Fury vs. Oleksandr Usyk won't be able to call themselves undisputed champion for very long.

Fury defends his WBC belt against Usyk, who puts his WBA (super), IBF and WBO titles on the line in tonight's monstrous contest.

The Riyadh showdown will crown the first undisputed heavyweight champion since 1999 when Lennox Lewis toppled Evander Holyfield.

But Fury or Usyk will only have undisputed status for 'less than two weeks'.

According to boxing reporter Dan Rafael, the winner of the undisputed fight will be forced to vacate the IBF title shortly after their clash.

The IBF belt will be contested by overdue mandatory challenger Filip Hrgovic, who meets Daniel Dubois on June 1.

Hrgovic, unbeaten in 17 pro bouts, is set to compete in his first world title fight since leaving the amateur ranks, where he won a bronze medal at the 2016 Olympics.

The Croatian star boasts a big win over Zhilei Zhang as he returns to action against Dubois.

Once Fury or Usyk relinquishes the IBF strap, Hrgovic and Dubois will battle for the vacant world title.

And the winner of Hrgovic vs. Dubois is lined up to face Anthony Joshua in September.

Daniel Dubois and Filip Hrgovic go head-to-head. Image: Getty

Saudi Arabia chief Turki Alalshikh plans to match Joshua against the winner of Hrgovic vs. Dubois, which will be for the IBF title.

On The MMA Hour, Alalshikh said: "We have Joshua's fight in September, it all depends on the result of the Hrgovic fight [with Dubois].

"Because now we have locked Wilder to Jared Anderson in the card in Los Angeles."

Then, the winner of Joshua vs. Hrgovic/Dubois is set to face the winner of the contracted rematch between Fury and Usyk, which is tentatively scheduled for October.

So, the winner of Joshua vs. Hrgovic/Dubois will take on the winner of Fury vs. Usyk 2 in what will be another undisputed bout.