Sofia - Stan Wawrinka failed to reach his first final since returning after knee surgery as he was dumped out in the Sofia Open last four by Bosnian qualifier Mirza Basic on Saturday.
Three-time Grand Slam champion Wawrinka, who made his comeback from injury at the Australian Open last month, lost 7-6 (8/6), 6-4 to the world number 129, who will face Romanian Marius Copil in Sunday's final.
Wawrinka battled hard in the opening set, but saw Basic take a tie-break on his third set point, before the Swiss threw away a 4-2 lead in the second.
The 32-year-old will now turn his attentions to next week's ATP event in Rotterdam, where his compatriot and 20-time Grand Slam winner Roger Federer can become the oldest-ever world number one with a run to the semi-finals.
Wawrinka, who was beaten by American Tennys Sandgren in the Australian Open second round, could meet Federer in the Rotterdam last eight.