David Haye is returning to boxing three years after announcing his retirement and will fight Joe Fournier next month.
Haye has not fought since losing his second consecutive fight to Tony Bellew in 2018 and had most recently been managing his former rival Derek Chisora.
However, the former world heavyweight champion has been tempted back into the ring by Fournier, with the pair agreeing to an eight-round heavyweight fight at the Staples Center, Los Angeles on September 11.
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Haye said: "This is not a comeback, this is about teaching Joe Fournier there are levels to the boxing game.
"One must stay in their lane or risk getting flattened. There are certain things in life you can't buy - Joe 'The Billionaire' Fournier needs to learn that.
"I was the heavyweight champion of the world just 10 years ago. I'm only 40. Joe is delusional. Champions are born not bought."
The highlight of Haye's career came in 2009 when he won the WBA world heavyweight title by beating Nikolai Valuev.
Two years earlier, the Briton became world champion at cruiserweight with a win against Jean-Marc Mormeck.
While Haye retired with a 28-4 record, Fournier - known as 'The Billionaire' after making his fortune in the fitness and nightclub industries - has nine wins to his name after making his professional debut in 2015.
The 38-year-old was encouraged to take up boxing by Haye and was even signed to his opponent's Hayemaker label.
Fournier had five years out of the ring before making his return on the Jake Paul-Ben Askren undercard in April, when he beat musician Reykon.
But despite their vastly different boxing pedigrees, Fournier said: "I will shock the world and knockout David Haye.
"Unquestionably one of the hardest challenges of my life, but this is the perfect fight at the perfect time for me.
"Haye is riddled with injuries, 10 years past his prime. I was a late starter to this pro boxing game - now I'm coming into my fighting prime.
"I'm healthy, motivated and ready to prove a point to Haye that on September 11, I will be victorious."
The Haye-Fournier clash will take place on the undercard of Oscar De La Hoya's comeback against former UFC champion Vitor Belfort.
Other fights on the card include the bout between UFC champions Anderson Silva and Tito Oritz, as well as Andy Vences vs Jono Carroll.
The problem with being a billionaire nightclub owner whose friends are A-listers? Somewhere along the way, you lose yourself. When the party ends, everybody goes home and you are left alone. In some ways, you were always alone.
Joe Fournier does not want sympathy. "I created this monster," he tells Sky Sports. But this friend to the rich and famous who became a professional boxer is more complex than you realise.
"I was slipping back into bad habits," he says.
"Predominantly my businesses are bars, clubs and restaurants in all the fun places. It's easy for my mates to visit me in Mykonos - one beer becomes two, becomes three…
"That leads to negative behaviour. I can get depressed and not myself.
"Boxing is good for my mental health. Tyson Fury is a big advocate of this. It keeps me from drinking too much, keeps me healthy and gives me a goal."