Stefanos Tsitsipas suffered an embarrassing defeat to outsider Roman Safiullin in Marseille on Friday, going down in straight sets to the 163rd-ranked Russian qualifier 6-4, 6-4.
“It’s the greatest victory of my career,” Safiullin said.
“It was my first time in a quarter-final and the first time I have beaten a top 10 player.”
Safiullin will meet world No.9 Felix Auger-Aliassime of Canada in the last-four.
Auger-Aliassime needed six match points to see off Ilya Ivashka of Belarus 6-3, 6-4.
The other semi-final sees second seed Andrey Rublev up against France’s Benjamin Bonzi.