Russian giant Nikolay Valuev retained his WBA world heavyweight title after American challenger Jameel McCline had to retire because of a knee injury.
A stricken McCline was stretchered out of the ring
McCline fell over after throwing a right at the end of the third round and was unable to get back on his feet.
Doctors said the 36-year-old's left kneecap had popped out, forcing the fight to be ended on medical grounds.
"I'll be disappointed for the rest of my life," said McCline over boos from the crowd in Basel, Switzerland.
"I had a great chance. I could have beaten him. It is pretty likely that I will stop now."
Valuev said: "I'm very disappointed at how it ended. I know all the spectators expected this fight to last a bit longer."
The bout had been billed as boxing's heaviest, with the duo weighing a combined 42st 2lb.
The 7ft Russian tipped the scales at 322╝lbs (23st 1lb) and 6ft 6in McCline was recorded at 268╝lbs (19st 1lb).
McCline had previously suffered heavyweight world title defeats to Wladimir Klitschko in 2002 (WBO) and Chris Byrd in 2004 (IBF).