Anthony Joshua has admitted to health problems before losing his heavyweight title to Andy Ruiz Jr. on June 1 in New York.
The British superstar claimed a unanimous decision (118-110, 118-110, 119-109) to become a two-time champion on Saturday against Ruiz, but he revealed symptoms from an unnamed issue hindered his preparations before losing in Madison Square Garden.
"I had some issue with my health which I was going through for a long time.
"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?
"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."
Ruiz shocked the boxing world with a technical knockout in the seventh round during the first fight, with Joshua struggling in a disappointing night for the Brit.
The former Olympic champion was floored by the Mexican in the third after a shot to the temple, and Joshua never fully recovered before the referee stopped the fight.
Joshua's poor performance baffled fight fans, but AJ's latest comments appear to explain his strange demeanour in the ring during the only defeat of his professional career.
"After my check-ups it showed what the problem was and this is what you have to get sorted," Joshua added. "Even in this camp I had an operation done, but as I'd started training in June I had no issues."
The 30-year-old Brit didn't explain the details of his operation, but Joshua appeared healthy and mobile on Saturday, dancing around Ruiz and focused behind his jab.
The Watford-born fighter took back his titles in style in Saudi Arabia, producing a boxing clinic to give Ruiz the second defeat of his career.
Joshua once again holds the WBA, IBF, WBO and IBO heavyweight belts, and he could now face the winner of Deontay Wilder and Tyson Fury's expected rematch in 2020.