By Brian Homewood

BERNE (Reuters) - Vladimir Klitschko retained his world heavyweight titles when he knocked down challenger Tony Thompson in the sixth round of a one-sided contest on Saturday.

The 36-year-old Ukrainian, who knocked out Thompson when they met four years ago, extended his unbeaten run to 16 fights, his last defeat coming when he challenged Lamon Brewster for the WBO title in 2004.

Klitschko, who has won 58 and lost three of his professional bouts, holds the WBA, IBF, WBO, IBO titles while his older brother Vitali holds the WBC title.

Thompson was never in the contest and was floored for the first time in the fifth round with a big right hook.

The American, the mandatory challenger nominated by the IBF, survived the count but looked groggy and struggled to hang on until the bell.

Dr Steelhammer wasted no time in finishing off the 40-year-old in the next round, knocking him over again with a flurry of punches to the head, prompting the referee to stop the fight.

"I didn't have a doubt from the beginning of the fight that I would defend my titles," Klitschko told reporters.

"It was really difficult to land the punches. He's very sneaky, he sees the punches, he never loses eye contact which makes him super alert.

"I felt I had the strength, speed, condition and power," he added.

Thompson, who suffered his third defeat in 39 fights, praised his opponent.

"I came up short for the second time," the American said.

"Vladimir is a great champion and his physical attributes showed in the ring tonight.

"I really feel I can beat any other heavyweight in the world, but if you can't be the champions..."

(Editing by Patrick Johnston)