LONDON: Anthony Joshua could face his "toughest-ever fight" when he defends the WBA, IBF and WBO heavyweight titles against Oleksandr Usyk in London on Saturday, the British boxer's promoter Eddie Hearn has said.

Joshua, 31, has height and reach advantages and will almost certainly be heavier at the Tottenham Hotspur Stadium but Hearn said he was nervous ahead of the bout.

"This is probably AJ's toughest-ever fight and I think he's going to prove to the world how good he is," Hearn told the PA news agency. "He's ready for this fight but it's the ultimate challenge with someone that's very strong mentally.

"Mentally it will be draining (for Joshua) because he's going to have to think a lot, there will be a lot of feints from both men and reactions are going to be key.

"I just can't wait to watch it unfold. It's a real, real high-end heavyweight fight. (Usyk is) going to be a tough, tough nut to crack. AJ's got to break him down, beat him up and hopefully take him out."

Usyk, 34, a former undisputed cruiserweight world champion, won heavyweight gold at the London 2012 Olympics while Joshua triumphed in the super-heavyweight division.

Despite the stiff test offered by Usyk, Hearn has been impressed by Joshua's confident demeanour in the build-up the fight, expected to draw a crowd of more than 60,000.

"I've never seen him so assured about a fight, which is ironic because technically this is his toughest fight," said the promoter. "He knows how tough this fight is, he wants to prove to the world that he's the best heavyweight in the world."

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Joshua is taking on his WBO mandatory challenger after a potential bout with Tyson Fury, the British WBC champion, fell through.

It was announced on Tuesday that Joshua had signed a "career-long" promotional deal with Hearn's Matchroom Boxing organisation.

Joshua's career has been overseen by Hearn since he turned professional eight years ago and although the Usyk fight was understood to be the last on the existing contract, the two parties have come to a new arrangement.

"We've never had one argument, we've never had one dispute, in eight brilliant years," said Hearn. "So if it's not broke then don't fix it and we're very proud that he's extended his contract with us for the rest of his career."