Hamburg, Germany -- (Sports Network) - Wladimir Klitschko knocked out Tony Thompson in the 11th round on Saturday at Color Line Arena, retaining his WBO, IBF, and IBO heavyweight belts.

The 6-foot-5 Thompson (31-2), an unorthodox southpaw from Washington, DC, gave Klitschko (51-3, 45 KOs) a challenge in the middle rounds, but the power punches belonged to the 6-foot-6 1/2 Ukrainian, who had the decision locked up before he knocked Thompson out at 1:22 of the 11th round.

As the American challenger tried to land a left hook in the 11th, Klitschko ducked and slammed him with a right to the chin, and Thompson collapsed to the canvas. He stumbled up towards the end of the 10-count, but referee Joe Cortez ruled it a knockout and Klitschko retained his belts.

Thompson had never been knocked out before, but he had also never faced a fighter of Klitschko's caliber. Klitschko has won nine consecutive bouts since suffering a technical knockout to Lamon Brewster in 2004.

In red and gold trunks, Klitschko had trouble with Thompson's unorthodox style early. The Ukrainian gained the upper hand early in the second round, though, as he forced Thompson from the middle of the ring into the corner with right- left combos. Both fighters drew blood over the right eye of the other in the second round.

Thompson, in oversized red, white, and blue trunks, battered the Ukrainian with body shots in the fourth round. After giving Klitschko some trouble in the middle rounds, Thompson appeared to run out of gas in round 7. He stopped attacking, and Klitschko picked him apart, capped by racking Thompson with a big right shot as the final seconds ticked away, further opening the cut over Thompson's eye.

Klitschko, a prohibitive favorite, dominated down the stretch before ending it with a right hook in the 11th.