Wladimir Klitschko insists Anthony Joshua is of a "totally different calibre" to his previous British opponents, David Haye and Tyson Fury.
Klitschko's lengthy reign as the heavyweight champion of the world came to an end last November when he suffered a unanimous points defeat to Fury in Dusseldorf.
Yet although he lost to Fury, Klitschko feels IBF title-holder Joshua, who he will face at Wembley on April 29, will offer a stiffer challenge.
"Totally different calibre," the 40-year-old responded when asked if Joshua was unique among British fighters he has faced.
"He's a true professional. He wants to get better and I've seen these qualities while he was at my training camp two years ago.
"He's calm and quiet and he likes to learn and that's great qualities to have. So I'm definitely going to face a challenge that I've never faced before."
Wednesday's news conference to promote the fight was a notably respectful affair, as both men praised each other repeatedly.
"That's right - get used to it," added Klitschko.
"It's extraordinary; it's different than any other press conference that you are observing in the past and I'm happy about it, because we represent the sport and the sport needs to look good, clean and not a nuthouse, like it used to be in the past for many years. I'm really happy that we're giving a different picture.
"We show respect but it doesn't mean that we will be so friendly with each other in the ring."
Klitschko has not fought since losing his heavyweight belts to Fury, but Joshua does not believe that is certain to count against the Ukrainian.
"When you've been boxing so long, he's got so much muscle memory, so I don't think it matters that he's taken that amount of time off," said Joshua.
"It's not the fight, it's the training camp, you take so much internal damage, sometimes your body needs six months to eight months to really heal itself. So it may have been good for him to have that time off and the reason I'm saying these positive things is because the Klitschko I face on April 29, if I'm honest with you, I want the best and the fittest.
"Because when we go to war and, listen, I'm going to speak as if I've won that fight - when I win, I want it to be the best Klitschko that I've beaten."