The pair meet in Manchester on Saturday night in a championship clash to become the undisputed cruiserweight champion of the world.
Oleksandr Usyk currently holds all four titles and also has the Ring Magazine belt to his name.
He is also yet to lose a fight having won all 15 of his showdowns with 11 by knockout.
Bellew has spent most of his career as a light heavyweight contender but is 7-0 at cruiserweight.
This includes a pair of stoppage wins against David Haye.
Anthony Joshua has issued the Ukrainian a warning as he prepares to defend his titles.
“I rate all of the British boxers highly and they’re always stepping up to the plate,” Joshua told Sky Sports.
“It’s a fight that’s credible and I respect Bellew because after the David Haye fight it was a question of is he going to retire or is he going to carry on.
“He carried on and he could have taken a different route but he’s gone to fight the best.
“He’s going to lose weight and put himself through a gruelling training camp and bring them titles back home.
“Good luck to Bellew, we’re routing for you.
“Usyk’s a perfect boxer, I hope Bellew’s working on his counter punches as well because that’s the way to knock him out I think.
“Let him tippy tap and get his rhythm and just kind of explode.
“As we can see Bellew can counter punch and he’s got a dirty left hook on him.
“He’s straight talking because he’s a champion and he’s done it at cruiserweight and he’s undisputed but the thing with boxing from middleweight up is that it can only take one punch.”