Anthony Joshua lost all three of his IBF, WBA and WBO heavyweight titles after being stopped in the seventh round by huge outsider Andy Ruiz Jr at Madison Square Garden in New York.
In one of the biggest upsets in heavyweight history, Joshua (22-1, 21 KOs) was sent to the canvas four times by the 10/1 challenger as his American dream turned into a nightmare.
Brooklyn's Jarrell Miller had initially been chosen as Joshua's opponent for his latest title defence until a six-month suspension ruled him out and gave Ruiz Jr, who had been widely mocked for his weight and shape, an unexpected opportunity with little over a month's notice.
When the Mexican-American (33-1, 22KOs) was knocked down in the third round, he appeared on the verge of losing as routinely as Joshua's previous 22 opponents, but he immediately responded with two knockdowns that left the British star fortunate to survive.
If Joshua appeared to recover throughout the fourth and fifth, in the sixth he again became hesitant even if there was little sign that he was about to be stopped.
Two further knockdowns in the seventh again left him fighting for survival, and though he returned to his feet after each, he was no longer able to defend himself, forcing referee Mike Griffin's intervention while he was on his still on his feet after one minute and 27 seconds.
Speaking in the ring after his victory, Ruiz Jr said: "I can't believe I just made my dream come true, the sky is the limit. That was the first time I've been down on the floor and it just made me stronger.
"I'm a Mexican warrior, I have that blood in me. I just made history for Mexico."
Joshua gave credit to Ruiz Jr, saying: "Respect to Andy Ruiz Jr. Sorry I let my fans down."
Speaking to BBC 5 Live, Joshua's promoter Eddie Hearn said he was shocked by the outcome.
"AJ got sloppy," he said. "He got sloppy, he got hit on the top of the head and he never really recovered.
"There were times in the fifth and sixth round that I felt he did recover and then again got caught and seemed to run out of gas.
"The night belongs to Andy Ruiz. It goes down as one of the big upsets in heavyweight history."
Hearn said Joshua would now focus on "rebuilding" and preparing for a re-match in November or December.
"This will devastate him," he said. "He will come back. It's now down to the re-match and winning that fight.
"He must win that fight and he will do anything he can to regain his title."
He added: "To get back to the heights he has been, he must win that re-match."