BERLIN — Wladimir Klitschko knocked out former sparring partner Tony Thompson in the 11th round to successfully defend his heavyweight titles tonight.

Klitschko (51-3, 45 knockouts) turned boos to cheers, sending the American to the canvas with a right hand 1:38 into the 11th. He redeemed himself after an ugly 10th in which he pushed Thompson to the canvas and fell on him.

Klitschko's ninth straight win was tougher than expected, as he had to survive a second-round head-butt that left both fighters bleeding at the right eye.

"I'm still world champion, but it's about three titles now— that means everybody that fights me is extra motivated," said Klitschko, who holds the IBF and minor WBO and IBO titles. "I haven't had a black eye for a long time, now I really look like a fighter."

His corner stemmed the cut, but his left eye was swollen after Thompson caught him with several good shots, especially a right hook in the fifth round.

But Klitschko was ahead on the cards and caught Thompson in the sixth with two good rights to take control of the fight.

He tired after the seventh — until he charged out in the 11th round and landed the knockout punch. That led the sellout crowd of 15,000 to chant "Klitschko, Klitschko" again.

"They were both tired, but Wladimir has experience now, then he came back with a second wind," Klitschko trainer Emanuel Steward said.

Thompson (31-2, 19 KOs) was a WBO mandatory fight for Klitschko. He served as Klitschko's sparring partner before one of the Ukrainian's worst losses, a two-round stoppage by Corrie Sanders in 2003.

Thompson's 27-fight unbeaten streak dating to 2000 was snapped. When Klitschko fell over him, Thompson's knee was hurt. However, the American said that was temporary and didn't lead to the knockout.

"I was fatigued, I thought he was fatigued too," he said. "He did what a great champion did, he took advantage when I was vulnerable. The only thing that hurts on me is my heart — for losing."

Klitschko, who started his career in Hamburg, fought in the city for the first time in eight years.

On the undercard, American Jonathan Banks (19-0-0, 14 knockouts) won a decision against Italy's Vincenzo Rossitto (36-5-4, 22 knockouts) to claim the vacant IBO cruiserweight title.