Alexander Povetkin says Anthony Joshua is better than Wladimir Klitschko in his prime and Deontay Wilder.
Povetkin was floored four times and defeated unanimously on points when he challenged Ukrainian Klitschko for the WBA, IBF and WBO world heavyweight titles five years ago.
The Russian get another chance to win the same three belts when he challenges Joshua (21-0, 20 KOs) in front of an expected crowd of 80,000 at Wembley Stadium, London, on Saturday.
Klitschko ruled for nine-and-a-half years before losing his belts to Joshua's fellow Briton Tyson Fury on points in a big upset in November 2015. Joshua, 28, is only making a sixth defence but Povetkin (34-1, 24 KOs), 39, considers him a more dangerous opponent than when he took on Ukrainian Klitschko.
"I would definitely say Joshua is a bigger challenge than Klitschko," Povetkin said.
"Since the fight with Klitschko a lot of time has passed, and I can assure you that I've become stronger, I've become wiser, I've become smarter, so I will definitely treat this fight in a different manner, and more professionally."
Like Joshua, Povetkin is an Olympic gold medalist who has compiled eight successive wins -- six by KO -- since that setback to Klitschko.
He lost out on another title shot, against American Deontay Wilder, in 2016 after he twice failed dope tests.
Povetkin continued his career without a ban stopping him from boxing and on March 31 viciously knocked out England's David Price. He enters the Joshua fight as the WBA mandatory challenger and says Joshua is the man to beat in the division.
"Both Joshua and Wilder are dangerous, but the only way to prove it is to see their fight. But my personal opinion if you talk about technique is that it is Anthony Joshua," Povetkin said.