Wladimir Klitschko stopped Tony Thompson in the sixth round to keep a comfortable hold on his WBA, IBF and WBO heavyweight title belts on Saturday in Bern, Switzerland.

Klitschko put Thompson on the canvas with a big right hand near the end of the fifth round at Stade de Suisse, and the 40-year-old American southpaw never recovered.

The Ukrainian champion dropped Thompson again in the sixth, and referee Sam Williams gave the challenger a standing count before stopping the fight at 2:56.

"Thompson was not so easy to box. I'm satisfied," said the 36-year-old Klitschko, who improved his record to 58-3 (51 KO).

Klitschko finished the fight five rounds sooner than when knocking out Thompson in the 11th of their first title fight in 2008.

Thompson, who was the mandatory challenger nominated by the IBF, fell to 36-3 (24 KO).

After the fight, Klitschko led the 22,000 crowd in signing "Happy Birthday" to his trainer Emanuel Steward, whose was 68 on Saturday.

"Tony Thompson is very hard to hit, " Steward said. "Tony was watching Wladimir's right hand all night."

A cagey opening round was followed by a scrappy second, as Klitschko twice wrestled Thompson to the floor.

Thompson briefly stepped up his cautious tactics when landing a left to Klitschko's face in the third.

Klitschko finally landed a long, straight right in the fifth and the round ended with Thompson struggling on the ropes.

The champion cornered Thompson toward the end of the sixth, and landed punches to the head though no single blow appeared to cause serious damage.

"I got caught but I'm OK," Thompson said in the ring. "He's strong and he's world heavyweight champion for a reason."

Klitschko extended the domination of the heavyweight ranks he shares with his elder brother Vitali.

Vitali was in his brother's corner as usual, and acted as cheerleader in the third round, urging on fans in the football stadium when the first chants of "Klitschko, Klistchko" stirred.

The elder Klitschko will defend his WBC title against Manuel Charr of Germany in Moscow on Sept. 8.