Oleksandr Usyk will face kickboxer Rico Verhoeven for the WBC heavyweight title in May in Egypt.
Two-time undisputed heavyweight champion Usyk will box Verhoeven in Giza on May 23.
In 2024 when he beat Tyson Fury, Usyk became the first undisputed heavyweight world champion of boxing's four-belt era, and the sport's first undisputed heavyweight king since Lennox Lewis.
He repeated a victory over Fury in their rematch and then knocked out Daniel Dubois at Wembley Stadium last year to reunify the IBF, WBC, WBO and WBA heavyweight titles.
The Ukrainian hasn't boxed since that victory over Dubois in July and vacated the WBO title.
Usyk said: "I truly respect people who reach the very top in their sport. Rico is one of them - a powerful athlete and a great champion. Being a champion isn't just about belts. It's about years of hard work, discipline, and belief.
"I respect his journey - he's truly the 'King of Kickboxing'. But this is boxing - a different game, with its own rules and its own kings. I'm ready and really looking forward to meeting him in the ring. It's going to be a unique experience for both of us, and I know the fans are excited too. A big night is coming."
Verhoeven is widely regarded as the best heavyweight kickboxer in the world and dominated the sport before vacating his crown after a remarkable championship reign in the Glory promotion that dated back to 2013.
The 36-year-old does have some boxing experience, he's trained with coach Peter Fury and in the past sparred with Tyson Fury. He's had one professional boxing bout, a 2014 knockout victory over Janos Finfera, who had a winless 0-5 record.
Verhoeven declared: "I spent 12 years as the undisputed heavyweight kickboxing champion and accomplished everything I set out to accomplish.
"But staying at the top for that long didn't take away the hunger, it strengthened it. I wasn't looking for comfort, so I started looking for the highest challenge available in another world.
"Usyk is undisputed in boxing. That's the kind of challenge that motivates me. Undisputed versus undisputed. The best facing the best."
There are precedents for a crossover fight like this. Both Tyson Fury and Anthony Joshua for instance have boxed UFC heavyweight champion Francis Ngannou though neither of those contests was for a world championship.
The WBC though have granted Usyk permission to make a voluntary defence of their title before he has to meet their mandatory challenger, which will be the holder of the WBC Interim title, Agit Kabayel.
Britain's Fabio Wardley had been the mandatory challenger for Usyk when he won the WBO Interim belt. When Usyk vacated the WBO title, Wardley was elevated and became their full world champion.
Wardley previously told Your Site: "I'm hugely disappointed. As proven by this and other fights I want to challenge myself against the best. I want to be in there with some of the best in the world."
Speaking of Verhoeven, he said: "I don't feel like it's a real, genuine challenge nor someone that deserves the shot.
"But I do understand or do concede Usyk is at a point in his career, he has earned the point where he's able to do what he wants for a bit in that sense. He's earned the position to, in that sense, take an easy fight."
