Wladimir Klitschko lost his WBO heavyweight title after being knocked out 27 seconds into the second round by Corrie Sanders.
It was a Sanders' left hand that did the damage
The unheralded South African floored Klitschko with a short hard left late in the first round, then sent him down to the canvas three more times.
Klitschko was defending his title for the sixth time.
The 37-year-old Sanders, who had never been given a big fight, jumped into the arms of his trainer, apparently shocked and overjoyed by the upset.
"I gave myself big chances to beat him - I knew I could do it," Sanders said.
Sanders, 39-2 with 29 knockouts, did exactly what pundits had predicted.
After a slow opening, he went after the 6ft 7in heavyweight, having already knocked out 18 opponents in the first round.
"I wasn't planning that - it just happened that way during the fight," Sanders said.
Ukrainian Klitschko had just signed a nine-fight bout with HBO and had hoped to sell himself as the next great heavyweight in the United States.
"I'm certain I will come back, I'm certain I will be world champion," Klitschko said.
"All great champions have been beaten, then came back. It happened to Ali, it's happened to Lennox Lewis too."
The younger of the Klitschko brothers - the eldest, Vitali, is a top ranked contender - suffered his second defeat, against 40 wins.