Anthony Joshua has claimed he was suffering from a mystery 'health issue' in his first fight with Andy Ruiz Jr.
Joshua was beaten by Ruiz Jr in June as he lost his unbeaten record and world heavyweight titles in a shock result.
But the Brit turned in an accomplished display in Saudi Arabia as he became a two-time champion.
He has refused to make any complaints for his defeat at Madison Square Gardens with a series of conspiracy theories emerging.
But has now opened up on his condition before the first fight - and admitted he needed surgery during camp for the rematch.
"I had some issue with my health which I was going through for a long time," he told BBC Radio 5 live.
"I didn't know what was wrong with me. I felt so tired and drained and thought it must be down to training.
"In the changing room before the fight I got a bucket of ice and was putting my head in it thinking 'why do I feel so tired?'
"After my check-ups it showed what the problem was and this is what you have to get sorted.
"Even in this camp I had an operation done but as I'd started training in June I had no issues."
Joshua did not discuss what procedure he went through but insists he feels refreshed after the victory in Saudi Arabia.
"The responsibilities of being world champion are difficult. All that stuff, feeling so tired, dealing with obligations.
"Now I have energy, I haven't missed a session."
Speaking after the fight, promoter Eddie Hearn set his sights on the Brit becoming undisputed.
The Brit reclaims his IBF, WBA and WBO titles with the victory but Hearn has now set his sights on Deontay Wilder .
Wilder holds the WBC crown and the Matchroom chief insists that belt will soon be Joshua's.
“He wants to be the undisputed champion, and he will be. I promise you that," Hearn told BBC Radio 5 live.
