BERN, Switzerland (AP) — Wladimir Klitschko stopped Tony Thompson in the sixth round to keep a comfortable hold on his WBA, IBF and WBO heavyweight titles Saturday night.

The 36-year-old champion Klitschko, 58-3 with 51 knockouts, floored Thompson with a big right hand near the end of the fifth round at Stade de Suisse, and the 40-year-old American 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,” Klitschko said.

In 2008, Klitschko stopped Thompson in the 11th round.

Thompson, 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 trainer Emanuel Steward, who turned 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 older brother, Vitali.

Vitali was in his brother’s corner as usual, and acted as cheerleader in the third round, urging on fans in the soccer 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.