David Goffin consigned world number one Andy Murray to his first defeat in three months and will face Rafael Nadal in the Mubadala World Tennis Championship final.
Murray won five consecutive tournaments to rise to the top of the rankings for the first time at the end of last season, but went down 7-6 (7-4) 6-4 to Goffin at the exhibition tournament in Abu Dhabi on Friday.
Playing his first match since beating Novak Djokovic in an ATP World Tour Finals showdown last month, Murray led 4-2 in the second set having lost the opener on a tie-break after neither player managed a break.
World number 11 Goffin, who had never beaten Murray in their previous five meetings, came roaring back, reeling off four successive games to beat the Wimbledon champion in straight sets.
Murray, given a bye into the last four, had won 24 consecutive matches before finally coming unstuck for the first time since Juan Martin del Potro got the better of the three-time grand slam champion in the Davis Cup three months ago.
Defending champion Nadal prevailed in the second semi-final, beating Milos Raonic 6-1 3-6 6-3.
The fit-again Spaniard, who recently employed Raonic's former coach Carlos Moya, raised his game in the deciding set after being pegged back in the second to down the world number three.
Jo-Wilfried Tsonga sealed fifth place courtesy of a victory over Thomas Berdych.