World heavyweight champion Wladimir Klitschko towers over challenger Alex Leapai during Wednesday's news conference in Berlin. Their fight goes Saturday in Oberhausen, Germany. Patrik Stollarz / AFP

Heavyweight champion Wladimir Klitschko has dubbed Alex Leapai "Australia's Rocky" ahead of Saturday's world title bout, when the challenger will try to complete an unlikely rags-to-riches saga.

Leapai sat behind bars in the high-security wing of a north Brisbane jail around the time Klitschko won his first world title in 2006, but now the 34-year-old will challenge for the champion's WBA, IBF, WBO and IBO belts in Oberhausen, Germany.

A bar fight saw the Samoan-born Leapai jailed for six months - "a time that still haunts me today," he said on Wednesday.

What he gives away in height (6 inches), he is determined to make up for with passion and commitment.

"I'm here to make history," said Leapai. "I want people in 100 years to remember that a Samoan-born Aussie came out of nowhere and beat one of the best heavyweights of all time.

"It's been a long road to get to this stage, but I will make sure the long ride has been worth it."

He earned the right to fight Klitschko by defeating WBO mandatory challenger Denis Boystov in Germany last November and now has the chance to become Australia's first world heavyweight champion.

Leapai has a 30-4-3 career record since turning professional in 2004, while the 38-year-old Klitschko is 61-3, with 51 KOs.

Klitschko's last defeat was on April 10, 2004, when he was stopped by Lamon Brewster in Las Vegas.

Leapai is the overwhelming underdog, but at 247 pounds, he has the size to at least trouble the champion.

"He's the underdog - but he can also bite," Klitschko told the German daily Bild. "Leapai will try to beat me with pure violence."

Still, the champion questioned Leapai's technique and tactics, adding: "He's Australia's Rocky. He's lacking the experience, and I'll use that against him."

But the Australian is determined to knock the Ukrainian champ from the top of world boxing.

"I will prove that he is just a man like any other, with two legs and a heartbeat," said Leapai.

"It's time for a change of ruler in the heavyweight division, and I'm the type of fighter everyone has been waiting for to do that."

Leapai was advised how to win by a disgruntled former sparring partner of Klitschko.

Sherman Williams told German daily Die Welt that he discovered during training camp that the champion hates taking body shots. Williams subsequently advised Leapai to exploit that.

"Hit him with lefts and rights to the body and then hit him on the chin with the right," said Williams.

"I guarantee that if Alex fights the way I tell him to, Klitschko will throw his arms around him like an octopus - and Australia will have a new world champion."

Leapai said he will be inspired by ANZAC Day, when Australia and New Zealand commemorate their soldiers killed in battle. It falls on Sunday.

But Klitschko said he has no intention of relinquishing his belts and that he wants to fight for at least another four years to break Joe Louis' world record of 12 years as the longest reigning heavyweight champion.

"I'll demonstrate that I'm the strongest boxer in the world. My mission isn't over yet," said Klitschko.