Two-time champion Gilles Simon stunned top seed and world number five Daniil Medvedev 6-4, 6-0 in just 67 minutes 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 in 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.
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