Anthony Joshua will defend his WBA, IBF and WBO heavyweight titles against Andy Ruiz Jr on June 1 at Madison Square Garden.
Joshua had been left in the lurch ahead of his United States bow after it emerged a couple of weeks ago that American foe Jarrell Miller had reportedly failed multiple drug tests.
The likes of Luis Ortiz and Michael Hunter had been linked with stepping into the breach but it is Ruiz Jr who will fill in as a replacement with a month's official notice.
The Californian-born 29-year-old has won 32 of his 33 professional contests, with his only setback a razor-thin majority points defeat to Joshua's former opponent Joseph Parker in December 2016.
Ruiz Jr said: "The chills - I'm really excited for this fight. There's nervousness in me, but they're happy nerves.
"This is my chance to make history, I want to be one of those greats like (Julio Cesar) Chavez, (Mike) Tyson, (Evander) Holyfield, Lennox Lewis. I want to be in that category."
At 6ft 2in, Ruiz Jr will be giving away four inches in height and many more in reach but he is bullish about his prospects of causing a seismic upset.
Ruiz Jr, who defeated Alexander Dimitrenko on April 20, added: "I think the fight is going toe-to-toe, two guys smashing each other's faces. I'm going in there to throw combinations like I've never done before, to improve my speed.
"A lot of people underestimated me, and I'm used to that. My whole life people underrated me so I'm just going in there to take all.
"I'm not going in there scared and I'm not going in there nervous, I'm going to go in there mad and to take what's mine.
"I think being so tall he fights like a big robot. I think with my style, my speed, my movement, I don't think he's fought anybody like me. It's going to be a whole different ball game.
"All the guys that he's fought, they usually run around from him. I don't think he's good going back. I'm going to bring the pressure, the speed, and the combinations to him.
"When I do pull out this win, everything is going to change. It's going to mean everything."
Joshua - who has won all 22 of his contests in the paid ranks and all but one inside the distance - is unfazed by the change in opponent.
The Briton said: "Ruiz is a different kind of challenge but one I embrace.
"We have worked in the boxing gym week on week and whomever is put in front of me at Madison Square Garden on June 1 will be dispatched in style. The rent will be collected."
Joshua's promoter Eddie Hearn feels Ruiz Jr will present a harder challenge for his charge than Miller.
Hearn said: "In my opinion, this is a tougher test than Jarrell Miller. Andy punches harder and is much faster - this is going to be a war.
"Andy will meet the best Heavyweight in the world on June head on at the Garden. Do not blink!"
Amir Khan's dream of finally proving himself one of the finest fighters in the world ended in a controversial defeat when he was stopped in the sixth round after a low blow by the great Terence Crawford at Madison Square Garden in New York.