Klitschko, taller with a longer reach, controlled the fight at will, scoring with left jabs and straight rights with hardly any opposition from the Samoan-born Australian.
Leapai went down when he was hit with a left-right combination. He got up but Klitschko put him away for good with 58 seconds left in the fifth.
“Glory to Ukraine,” Klitschko said after his one-sided win.
Klitschko recorded his 53rd KO in 62 wins, with three defeats. Leapai dropped to 30-5, with three draws. In his 25th world championship fight, Klitschko retained his WBA and IBF heavyweight belts, plus the minor WBO and IBO versions.
Leapai, the first Australian challenger in 106 years, never had a chance. The only time he ever got close to the bigger Ukrainian was just before getting floored in the fifth.
He took an eight count in the first round, although he claimed to have slipped.
Leapai’s only chance was to try to get inside, but Klitschko easily stopped him with his effective jab. Leapai appeared to finally hit Klitschko in the fifth but the champion replied with a barrage and put the Australian down. Leapai got up but was clearly shaken, and Klitschko finished him off with a huge right to the jaw.
“It wasn’t easy, my head was in Ukraine and what is going on there. I hope there is no war and people dying. I am proud of my people,” Klitschko said.
Klitschko’s elder brother, Vitali, who has retired from boxing to run for political office in Kiev and who has been one of the leading opposition figures in Ukraine, was in Wladimir’s corner as usual.
“It was textbook fighting by Wladimir. He did not make a mistake,” Vitali said.
Leapai, a delivery truck driver, said it was time to “go back to the gym.”
“I tried to take the fight to him and it didn’t work. I am all right, I was waiting for my opportunity but he is a champion and a great fighter,” Leapai said.