BERLIN -- American Chris Byrd won the WBO heavyweight title Saturday when Ukrainian Vitali Klitschko stopped fighting in the 10th round because of a shoulder injury.

It was a major upset for Byrd. Klitschko had won all of his previous 27 professional fights inside the distance.

"I want to apologize to all my fans, but I have a sore left shoulder and I couldn't go on," Klitschko said after losing the lightly regarded WBO title he won from Britain's Herbie Hide last June.

"It had pain since the third round. I hit with heavy pain," Klitschko said.

Byrd, 31-1, was a late substitute for Donovan "Razor" Ruddock, who pulled out because of a stomach muscle injury.

"I myself didn't expect to win," Byrd said.

Byrd, the 1992 Olympic silver medalist, gave away 34 pounds and six inches to the 6-foot-8 Klitschko, who appeared to be completely puzzled by the American's ducking and bobbing.

Before his injury forced him to quit, Klitschko, 28, had experienced problems against the left-handed Byrd. Klitschko was never really in trouble but could not exploit his huge advantage in reach against his smaller but more mobile opponent.

By the end of the fight, the crowd was cheering Byrd, who backed up Klitschko in several rounds with a flurry of punches and taunted him.

"I knew I had a chance," said Byrd. "Nobody wants to fight me because I make good fighters look bad."