Audley Harrison was outpointed by Belfast cabbie Martin Rogan at London's ExCeL Arena, casting doubts over the former Olympic champion's future.
Referee Ian John-Lewis scored the 10-round encounter 96-95.
The 37-year-old Harrison, a gold medallist in Sydney in 2000, has now suffered four losses as a professional.
Rogan, also 37, won the heavyweight Prizefighter tournament earlier this year and is unbeaten in 11 bouts, while Harrison falls to 23 wins in 27 fights.
Harrison landed freely with his right hand in the second round but Rogan turned things around with his crude style.
Rogan then stunned the Wembley southpaw with a left hook in the fourth and Harrison's negative tactics invited Rogan on.
Rogan connected with a good right uppercut in the seventh and had his man reeling with another in the eighth, prompting jeers from the crowd.
Despite his latest setback, Harrison afterwards insisted he would eventually find a way to "make it work".
He said: "The dream isn't over. I'm going to go back to the drawing board."
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