Ukraine's world heavyweight champion Vitali Klitschko has announced his retirement from boxing after a knee operation.
"Recently I've spent more time battling injuries than my opponents," the 34-year-old said in a statement. "It's a hard decision to take but I'd like to go out at the top and, with my departure, open the way for my successors."
Klitschko was forced to pull out of his World Boxing Council (WBC) title defence against Hasim Rahman, set for November 12, after injuring his right knee while sparring in Los Angeles last Thursday.
He had twice had to postpone fights with Rahman earlier this year because of thigh and back problems, and his latest injury left him under threat of being stripped of the title if he could not fight again within 90 days.
After having surgery to repair damaged knee ligaments on Tuesday that target became unrealistic, with his expected recovery time put at six months.
Rahman became WBC interim champion after beating Monte Barrett and is expected to be awarded the full title.
Klitschko has a record of 35 victories and two defeats, with 34 knockouts.
His purse for the scheduled November fight with Rahman was set at $7.8 million.
Klitschko and his younger brother Vladimir, also a leading heavyweight, made Germany their base for years and were worshipped in the country before moving on to the United States in a bid to further their careers.
Klitschko defeated South African Corrie Sanders in April 2004 to win the WBC title.
He won his first defence with a brutal eight-round stoppage of Briton Danny Williams in December to confirm his status as the world's top heavyweight but he has not fought since.
World Boxing Association (WBA) heavyweight champion John Ruiz of Puerto Rico earlier this year called for a tournament to unify the division.
The IBF champion is Chris Byrd, while fellow American Lamon Brewster holds the WBO belt.