Joshua hurled two of Usyk’s belts to the floor and stormed out of the ring in a fit of pique in Jeddah, then returned moments later to deliver a rambling speech peppered with expletives.
The 32-year-old’s career appears at a crossroads after Sunday’s second successive defeat to the unbeaten Ukrainian and he came under fire for his emotional post-fight reaction.
Writing on Twitter, Joshua hailed Usyk as “a class act”, adding: “Yesterday I had to mentally take myself into a dark place to compete for the championship belts!
“I had two fights, one with Usyk and one with my emotions, and both got the better of me.
“I’ll be the first to admit, I let myself down. I acted out of pure passion and emotion and when not controlled it ain’t great.
“I love this sport so so much and I’ll be better from this point on. Respect.”
Tony Bellew, a retired cruiserweight champion who was similarly beaten by Usyk, sprang to Joshua’s defence.
“Made a mistake like everyone else on planet earth has done, only difference is he makes his in front of millions on TV and then comes out (which is hard to do) and apologises to everyone!” the Briton tweeted.
The 6ft 6in (1.98m) Joshua scented victory when he had Usyk hanging on in the ninth round, only for the mobile southpaw to come firing back and control the final stages for a deserved split-decision win.
While Joshua was left trying to rebuild his career after his third defeat, Usyk retained the IBF, WBA and WBO titles and will now seek a unification showdown with WBC champion Tyson Fury.