Sofia - Australian Open semi-finalist Stefanos Tsitsipas was stunned in the quarter-finals of the Sofia Open on Friday, losing 6-3, 7-6 (7/5) to veteran Frenchman Gael Monfils.
Greece's world No 12 Tsitsipas defeated Roger Federer in the last-16 in Melbourne before falling to Rafael Nadal in the semi-finals of the season's opening Grand Slam event.
However, the 20-year-old came unstuck on Friday against the world No 33.
The 32-year-old Monfils next faces Russian third seed Daniil Medvedev who beat Martin Klizan of Slovakia 6-4, 6-1.
Saturday's other semi-final sees Italy's Matteo Berrettini facing Marton Fucsovics of Hungary.
Berrettini downed Spain's Fernando Verdasco 4-6, 7-5, 6-4 while Fucsovics progressed when Roberto Bautista Agut withdrew through injury.
Results from the ATP Sofia Open on Friday (x denotes seeding):
Matteo Berrettini (ITA) bt Fernando Verdasco (ESP x6) 4-6, 7-5, 6-4
Marton Fucsovics (HUN) bt Roberto Bautista (ESP x4) - walkover
Daniil Medvedev (RUS x3) bt Martin Klizan (SVK) 6-4, 6-1
Gael Monfils (FRA x7) bt Stefanos Tsitsipas (GRE x2) 6-3, 7-6 (7/5)