Tyson Fury claims fellow heavyweight and former world champion Anthony Joshua is "finished" in boxing.
Joshua lost his IBF, WBA and WBO titles in a shock defeat to Andy Ruiz Jr in New York on June 1.
After the fight, Fury had sent a message on Twitter that urged his fellow Briton to "rest up, recover, regroup and come again".
But this week he told BBC Sport: "When a man doesn't want to be there once, he will always do it and it's hard to come back from.
"He did it that night and I don't think he will come back from it. Finished. Ask any top trainer who has been around the sport a lifetime. When he got to the ring I saw he didn't want to be there.
"Everybody thought that man could beat me. In what world could he ever beat me? What were people seeing?
"I don't know what people were looking at as I don't know in what world he could have beaten me."
Joshua's seventh-round stoppage by Ruiz came as a huge surprise as his opponent had been given barely one month's notice for the fight and weighed in at over 20 pounds heavier than the Briton.
But Fury, who shed around 10 stone in a bid to reestablish himself as the dominant heavyweight in the world, insists body shape is no indicator for success.
"Physically Joshua could out-train everyone," added the 30-year-old, who is preparing for his Las Vegas debut fight against Tom Schwarz.
"He would break every heart monitor, has probably worn out every treadmill in the gym, smashes the bag, it's all very unimportant.
"Boxing isn't rocket science and today people are trying to make it a scientist thing. If you can fight you can fight, if you can't you can forget about it.
"You can eat every protein bar in the world, when you get banged on the chin and panic, it doesn't really help you much.
"The best feeling I got from it was the two best conditioned fighters in the last 20 years have been (Wladimir) Klitschko (who Fury beat in 2015) and Joshua and they both got done by two fat men."