Marseille - Two-time champion Gilles Simon stunned top seed and world No 5 Daniil Medvedev 6-4, 6-0 in just 67 minutes on Friday to reach the semi-finals in Marseille.
The 35-year-old Frenchman, who was champion at the tournament in 2007 and 2015, will face Canadian seventh seed Felix Auger-Aliassime for a place in the final after securing a third win in three meetings with Russia's Medvedev, the US Open runner-up last year.
Auger-Aliassime defeated Egor Gerasimov of Belarus 7-5, 6-2 to move to within one win of a fifth career final.
The 19-year-old Canadian was runner-up to Gael Monfils in Rotterdam earlier this month in what was his fourth defeat in four ATP finals.
Auger-Aliassime defeated Simon in their only previous meeting, a straight sets win in Stuttgart last season.
Defending champion and second seed Stefanos Tsitsipas of Greece made the last-four with a 7-5, 6-3 victory over Canada's Vasek Pospisil.
He will now play Alexander Bublik of Kazakhstan, who knocked out Canadian fourth seed Denis Shapovalov 7-5, 4-6, 6-3, for a place in Sunday's final.
World number 55 Bublik will be playing in his third career semi-final.
Results from ATP event in Marseille on Friday (x denotes seeding):
Gilles Simon (FRA) bt Daniil Medvedev (RUS x1) 6-4, 6-0
Felix Auger-Aliassime (CAN x7) bt Egor Gerasimov (BLR) 7-5, 6-2
Alexander Bublik (KAZ) bt Denis Shapovalov (CAN x4) 7-5, 4-6, 6-3
Stefanos Tsitsipas (GRE x2) bt Vasek Pospisil (CAN) 7-5, 6-3