Wladimir Klitschko should be back in the ring just over three months after recording his 52nd knockout victory.

Wladimir Klitschko now has to defend his WBA heavyweight title against Alexander Povetkin after knocking out Francesco Pianeta on Saturday night.

Klitschko has been ordered by the WBA to face the Russian in Moscow on August 31.

Povetkin is the WBA’s “regular” champion and Klitschko the “super” champion since he beat David Haye in 2011 to add the WBA belt to his others titles.

Now Klitschko and Povetkin are obliged to fight to leave just a single WBA champion. The two former Olympic champions were twice due to clash but Povetkin pulled because of injuries.

Klitschko retained his WBA, WBO, IBF and IBO belts against Pianeta in Mannheim, Germany, on Saturday night, winning by knockout in the sixth round.

German-Italian Pianeta, who survived testicular cancer three years ago, came into the fight undefeated after 29 fights, but was outclassed by the 37-year-old champion.

"I want to thank my opponent. There are a lot of challengers who talk a lot, but he got in the ring and fought bravely," said Klitschko.

His comments were a thinly-veiled dig at Haye, who has constantly tried to lure Klitschko back into the ring after losing to him in July 2011.

Pianeta fought on after hitting the canvas in the fourth round and the champion paid tribute to his courage. "After the battle he had against cancer he deserves respect," said Klitschko.

"He tried everything to hit me. If he had landed one of his big punches, I wouldn't have got up."

Pianeta made an ambitious start after a huge right over the top of Klitschko's guard failed to connect and the champion responded with two heavy jabs that found their target.

After the knockdown in the fourth round the challenger struggled to stay in the fight. He was down again in the fifth, but beat the count.

With his trainer ready to throw in the towel, Pianeta did not get up after being knocked down in the sixth round.

"I'm OK, just very disappointed and sad," Pianeta said after receiving treatment in the ring.

It was the 51st knockout of Klitschko’s career. The last of his three defeats was nearly a decade ago.

Russian promoter Vladimir Hryunov has won the purse bid to stage the bout against Povetkin, coming in with a staggering $23.33 million bid.

Klitschko is entitled to 75 per cent of the amount, which would give him a career-high $17,250,000 purse.