WBO, WBA, IBF, IBO heavyweight champion Anthony Joshua thought he was gearing up for a full division unification with WBC champion Tyson Fury - with the contest being targeted for the month of August in Saudi Arabia.

But the fight fell apart when an arbitrator ordered Fury to honor a rematch clause due to Deontay Wilder.

Fury was initially scheduled to face Wilder on July 24, but he was diagnosed with COVID-19 and the fight was pushed back to October 9.

The unification fallout forced Joshua to go in a different direction.

On Monday, it was officially announced that Joshua would make a mandatory defense against undefeated Oleksandr Usyk at the Tottenham Hotspur Stadium on September 25.

Usyk, who unified the entire cruiserweight division and captured Olympic gold in 2012, is viewed as a very dangerous opponent.

Joshua has a lot of respect for Usyk and views the contest as being just as difficult as a potential unification with Fury.

"I don't want to point any fingers. I was definitely ready. We move forward. I've got another challenger who's good, just as good [as Fury]," Joshua told Sky Sports.

"I have to be serious about him now. He will definitely step up, he will definitely take the fight. That's what I need - people that are serious. I would have loved to have boxed for the undisputed championship because it was a big fight for the public. We had everything set up. I have to move forward and not keep dwelling. 

"The most important fight for me is the next one but you have to have a vision of where you want to get to. I have to stay consistent and stay ready. Once I fight Usyk, who knows what will happen next? Will I fight Fury, Wilder, Dillian Whyte, Andy Ruiz, Luis Ortiz? Who knows. I have to stay on track and ready for each fight, but have a long-term plan. It's about the belts, not the person. I don't mind who it is. I want to fight for legacy."