Greek world number five Stefanos Tsitsipas saw his hopes of a third successive Marseille ATP title dashed by 93rd-ranked Pierre-Hugues Herbert in the quarter-finals on Friday.
Herbert, better known as a doubles specialist, swept past Australian Open semi-finalist Tsitsipas 6-7 (6/8), 6-4, 6-2.
The 29-year-old will next face fellow Frenchman Ugo Humbert who put out compatriot Arthur Rinderknech 4-6, 7-5, 7-6 (7/4).
"It's incredible, it's my first win against a top-five player," said Herbert.
"I knew that I'd have to play a great match to win and that's what I did."
Top seed Daniil Medvedev, who will replace Rafael Nadal as world number two on Monday, eased to a 6-2, 6-4 victory against 19-year-old Italian Jannik Sinner.
Medvedev, who has nine career titles, is through to his first semi-final in Marseille, after falling in the quarter-finals in 2017 and 2020.
"Jannik always tries to hit the ball as hard as he can," Medvedev told atptour.com.
"You cannot give him easy balls, otherwise you are going to run all over the place and he is probably going to hit a winner."
On Saturday, Medvedev will face Australian qualifier Mattew Ebden for a place in the final.
World number 287 Ebden upset 2018 champion Karen Khachanov of Russia, 4-6, 6-4, 6-2.