By Duncan Johnstone

World heavyweight champion Wladimir Klitschko has warned rising Kiwi boxer Joseph Parker he faces "a very great challenge" in Sherman Williams when they clash in Germany at the end of the month.

Parker and Williams fight on the undercard of Klitschko's title defence against Australian-based Samoan Alex Leapia in Oberhausen on April 27.

Klitschko has firsthand knowledge of Williams, using the veteran Bahamas fighter as one of his main sparring partners because of his physical resemblance to Leapai.

The durable Williams has impressed Klitcschko enough to believe Parker could be in for a tough night.

"Joseph Parker is going to face a very experienced and strong fighter in Williams," Klitschko said when attention switched to the impressive undercard as the giant Ukrainian did a media call for the promotion.

"Joseph is going to have a great challenge in the ring ... a very great challenge in the ring."

The 41-year-old Williams has been stopped just once in his 52 professional fights and has won 36 of them, 19 by knockouts.

Parker has just seven pro fights under his belt. He is unbeaten but hasn't fought more than six rounds with six knockouts coming inside three rounds.

Klitschko, who has been a childhood hero of Parker's, was happy to offer the 22-yea-old some advice but said, ultimately, only the New Zealander can help himself.

"I can speak a lot and I can say a lot of things but you have to gain your own experience because at the end of the day you are going to be alone in the ring," Klitschko said.

"What and who is going to say what?  You have to face your opponent and be okay with yourself.

"Definitely some times during the fight you've got to challenge yourself because it's going to get tough and sometimes you have to get up from the canvas and continue to fight to be successful and win it.

"It doesn't matter what I'm going to say - just stay focused and be real obsessed with what you're doing and be in love with that because if you don't like what you're doing you will never have success."

Klitschko said David Tua proved what could be done fighting out of New Zealand, but advised Parker to be his own man, rather than being bogged down by trying to be the next Tua.

"I think Joseph has the example with David Tua and David was an amazing fighter," Klitchko said.

"I think there are a lot of examples in the division and history that he can take, not just in his corner of this planet but I think that anyone can do it. Anyone can make it.

"At the end of the day it's face to face with another opponent in the ring and the stronger is going to win.

"But there are a lot of tools to take care of and the harder the preparation the easier the fight is. So prepare hard and then your fight is going to be relatively easy.

The Germany promotion is being broadcast in New Zealand on Sky's pay-per-view channel.