Undisputed heavyweight champion Wladimir Klitschko is currently unbeatable and improving with age, according to his brother Vitali, after Wladimir knocked out Kubrat Pulev to earn the 17th defense of his world titles.

The three-time former world heavyweight champion Vitali was in his younger brother's corner in Hamburg on Saturday as a sharp left hook from Wladimir put Pulev down for the fourth and final time after two ­minutes 11 ­seconds of the fifth round.

The 38-year-old Wladimir is inching closer to the legendary Joe Louis' record of 25 heavyweight defenses, but Vitali, currently the mayor of Kiev, says his sibling has no peers in the division.

"I don't see anyone in the world who can beat him," said the 43-year-old.

"Age is just making him stronger."

Wladimir completely dominated the fight in an impressive display to earn the 54th knockout and 63rd victory of his career with the last of his three defeats now more than a decade ago.

The 33-year-old Pulev suffered the first defeat after 20 straight victories and was made to pay for his pre-fight talk when he described Wladimir as behaving "like a girl" and "has no heart."

The Bulgarian was taken to hospital after the Hamburg bout with severe concussion and a suspected fracture of the right cheekbone from the sharp left hook which ended the fight.

Having been put down twice in the first round and once in the third before the knockout punch, Pulev sounded ­ridiculous when he said Wladimir had been "lucky."

"I have never seen such arrogance from an opponent and there was a price to pay for that," said Wladimir.

Wladimir jumped on a plane in the early hours of Sunday morning to Nashville, the US, where his girlfriend is expecting the couple's first baby next month.

Klitschko says he does not want to take a long break from boxing after the birth and his manager Bernd Boente said his next defense is scheduled for April in either Germany or the US.

Possible opponents are unbeaten American Bryant Jennings, the winner of the British dual on November 29 between Dereck Chisora and Tyson Fury or the winner of the World Boxing Council heavyweight title fight between champion Bermane Stiverne and mandatory challenger Deontay Wilder.