Tony Bellew says a fight with his "worst nightmare" Oleksandr Usyk would be the last of his career due to the "serious stick" he would take in a defeat of the history-making cruiserweight.

Usyk called Bellew out after beating Murat Gassiev on points in the World Boxing Super Series final in Moscow to become the first man to hold all four cruiserweight world titles at the same time.

Bellew accepted the challenge, having said he may retire after beating David Haye for a second time in May, and talks have been held over a title bout.

The Englishman is confident he would end the Ukrainian's unbeaten record and vowed that would be the last time he is seen in the ring.

He told Sky Sports: "I will ask a load more questions of him than he can of me, because I've been through every single scenario you can imagine in a boxing ring,

"I've got so much respect for him. I sparred with him in the amateurs and he was a master then. He is my worst nightmare, a tall, rangy southpaw, with skills to burn and fantastic footwork.

"But I can tell you now, if I fight Oleksandr Usyk, it will be my last ever fight, because I will take some serious stick.

"If I fight Tyson Fury next, I could have another fight after that because I will beat Tyson Fury and it would be much easier than beating Usyk, without a shadow of a doubt.

"I understand exactly what I am getting into. It's a very hard, dangerous fight. I might not be the same man coming out of it, but I can tell you, I will win. I don't know how, I don't know when it will happen, but I know I will take him."