A nervous Eddie Hearn expects a mentally "draining" fight for Anthony Joshua when he takes on Oleksandr Usyk in their highly-anticipated showdown at Tottenham Hotspur Stadium.
Joshua has height and reach advantages and will almost certainly be heavier when he defends his WBA, IBF and WBO heavyweight titles this weekend, but Hearn is among those to anticipate the toughest test of the Briton's career.
That is because of a breathtaking skill set possessed by the Ukrainian who won gold at London 2012 the day before Joshua in the division below, including deft footwork, blurring hand speed and stamina few others can match.
Usyk, a former undisputed world cruiserweight champion, is therefore regarded by Hearn as the most demanding of foes for Joshua, who has avenged a defeat to Andy Ruiz Jr and risen from the canvas to stop Wladimir Klitschko.
Promoter Hearn told the PA news agency: "I'm very nervous, which is great. I want to be nervous because I then know it's a real fight.
"This is probably AJ's toughest ever fight and I think he's going to prove to the world how good he is. 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."
While Hearn may be anxious at how the headline event will unfold on Saturday night, where more than 60,000 fans are set to be in attendance at north London, he has been impressed at Joshua's confident demeanour in the build-up.
Hearn said: "I've never seen him so assured about a fight which is ironic because technically this is his toughest fight. He knows how tough this fight is, he wants to prove to the world that he's the best heavyweight in the world."
Joshua is taking on his WBO mandatory because a bout against Tyson Fury fell through – despite months of protracted back and forth negotiations – after the WBC champion was ordered to defend his crown against Deontay Wilder.
It is hoped that if both Joshua and Fury, who fights Wilder for a third time on October 9, come through their respective latest tests then a mouthwatering clash between the two compatriots can be arranged next year.
Hearn, though, said: "Do you know what? At the moment it's the last thing on my mind because all I care about is winning on Saturday."
One issue has been settled this week after it was announced on Tuesday Joshua had signed a "career-long" promotional deal with Hearn's Matchroom Boxing.
Joshua's career has been overseen by Hearn since turning 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.
Hearn said: "It shows the quality of Matchroom and the job we've done as a team for him. He's very comfortable with us, he trusts us, that goes a long way for Anthony Joshua and he also has a tremendous amount of loyalty.
"We've never had one argument, we've never had one dispute, in eight brilliant years. 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."
While Hearn ended a long-term partnership earlier this year with broadcasters Sky Sports, moving to streaming service DAZN, it is his old employers who will be screening Joshua-Usyk in the UK and Ireland on pay-per-view.
Joshua will be a network-free agent after this weekend, but Hearn added: "People presume because of the deal that we'll bring him over to DAZN. At the end of the day we'll make the right decision for Anthony Joshua, not a decision that will benefit Matchroom."