Derek Chisora has said he will have to pull off one of the biggest wins of his career against Oleksandr Usyk if he is set up a world title shot against his friend Anthony Joshua.
Chisora (32-9, 23 KOs), 36, rates former undisputed world cruiserweight champion Usyk, (17-0, 13 KOs), 33, who he faces in London on Saturday, as better than the likes of former world heavyweight champions Vitali Klitschko and David Haye as well as reigning WBC world heavyweight champion Tyson Fury.
Fury beat Chisora in 2011 (unanimous points) and 2014 (retired, round 10), and is ranked No. 1 in the heavyweight division ahead of fellow Englishman Joshua, who holds the WBA, IBF and WBO belts, according to the latest ESPN rankings.
"I think Usyk is the top, number one fighter out of all the fighters I have faced because he's bringing movement, agility and picks his punches well," Chisora told ESPN.
"He moves about the ring well. I'm trying to find the right sparring for him and it's not been easy.
"It will be difficult to stop his movement, get to him, but it's what I have got to do."
Chisora, who moved to London from Zimbabwe in childhood, was unanimously outpointed by Klitschko for the WBC belt in 2012, before being stopped by Haye -- now his manager -- later that year.
Further defeats to current contenders Kubrat Pulev, who challenges Joshua on Dec. 12, and Dillian Whyte followed, but the entertaining London-based boxer revived his career with three wins last year to earn a shot at Ukrainian Usyk, who is the WBO's mandatory challenger for Joshua.
Chisora hopes beating Usyk will see him inherit the WBO mandatory shot at Joshua, who started boxing at the same boxing gym in north London where Chisora used to train.
Joshua made rapid progress after starting at Finchley ABC and won a gold medal at super-heavyweight at the 2012 Olympics, the same Games Usyk won a gold at heavyweight.
"We are friends and very good friends at that," he said.
"I did some sparring with him up in Sheffield before the Andy Ruiz fight [last year].
"If we ever fight, it could be strange, but I just look at it as business and after we will hug each other and be friends again.
"I left Finchley before AJ was properly there, but I saw him first when he was being coached by Sean Murphy. He was the first guy in the gym and he wanted it more than anyone else, I remember that. He was only a novice back then. I saw him when he was first there, and he made rapid progress."