Tyson Fury is counting down the days as he targets "redemption" against Oleksandr Usyk after resuming training following the only defeat of his career.

Fury, 35, and Usyk, 37, will lock horns again on December 21 in Saudi Arabia after the Ukrainian became undisputed world heavyweight champion by winning a split decision in their first meeting in the Gulf kingdom last month.

Speaking for the first time since their clash, Fury published footage on social media of him hitting a punch bag before vowing to take revenge.

"Hey guys, Tyson Fury here. Back in the gym as you can see," he said on his Instagram account.

"Counting down the days until I can get my redemption on Usyk. December the 21st is going to be my time.

"I've done everything in boxing, but I ain't come back from a loss. And I relish it, the chance to come back and redeem myself. Get up! December 21st is all me. Undisputed here we come."

In a caption accompanying the post, Fury said he knows the reasons for his defeat by Usyk, who now holds the WBC, WBA, WBO and IBF belts.

"The path to redemption is all mine. I know what I have to do, I saw my shortcomings and best believe I am inevitable," he said.

Britain's Lennox Lewis was the last man to unify the major heavyweight belts -- three at the time -- after beating Evander Holyfield in 1999. 

By the time Fury and Usyk collide in December, Usyk may have been stripped of the IBF belt for his failure to meet its mandatory challenger.

That would mean the undisputed title would not be on the line.