Wladimir Klitschko will defend his WBA, IBF, WBO and IBO heavyweight titles against Australian challenger Alex Leapai in Oberhausen, Germany, on April 26.
Klitschko's management company says the bout will be the Ukrainian fighter's 25th world championship fight.
The 1.83-meter (6-foot) Leapai defeated the previously unbeaten Denis Boytsov in November to become the WBO's mandatory challenger. The 34-year-old, who was born in Samoa, has a 30-4-3 record with 24 knockouts since turning professional in 2004.
Leapai says, "I'll beat Wladimir and it won't even be my toughest fight."
The 37-year-old Klitschko, who comfortably defeated Alexander Povetkin of Russia in Moscow last October, is 61-3 with 52 KOs.
Klitschko's older brother Vitali, the former WBC champion, stepped back from boxing to concentrate on politics in their native Ukraine.
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