World heavyweight champion Wladimir Klitschko knocked out Bulgarian challenger Kubrat Pulev with a fierce left hook in the fifth round on Saturday to retain his IBF title and end his opponent's unbeaten record.

The 38-year-old Klitschko, who also holds the WBO, IBO and WBA titles, knocked down the 33-year-old twice in the opening round as Pulev initially struggled to keep his taller opponent at a distance. The Bulgarian, who had won his previous 20 fights, was again floored in the third round when he was sent sprawling by a thundering left hook.

"I made my professional debut here 18 years ago and I love this city," a beaming Klitschko, who has his base in the northern German city, said in a ringside interview.

"It was difficult to get that hook in. I kept trying but he managed to stay away. I tried it a few times and at the end it worked. To Pulev’s fans I want to say, you have to be sporting, Pulev fought a good fight. It was hard to work, [to land] the left jab. Pulev prepared well.”

Klitschko has now improved his record to 63 wins out of his 66 fights with 54 knockouts. Pulev is 20-1.