Mahammadrasul Majidov was upset by Andrey Fedosov on Saturday night in Florida after sustaining a gruesome-looking ankle injury.
The heavyweight danger-man lost his unbeaten record in bizarre circumstances after twisting his ankle while falling backwards from a knock-down.
The damage was done when Fedosov landed a big right hand in the first round and Majidov fell on top of his own ankle.
He looked bewildered on the canvas and, when he bravely regained his footing, was clearly unstable.
Fedosov immediately pounced and floored him again to earn a surprise, and unusual, victory.
The injured Majidov was stretchered out of the ring.
Azerbaijan’s Majidov had won his first three pro fights impressively and was touted to be a threat in the heavyweight division but, aged 34 and with a serious injury to deal with, his future now looks uncertain.
He was a three-time world amateur champion who notoriously stopped a young Anthony Joshua at the 2011 world championships.
Russia’s Fedosov hadn’t boxed in two-and-a-half years but promoter Eddie Hearn tweeted after his unexpected win: “Winner stays on. Fedesov going to be in some fun fights!”