The 20-year-old, who beat Roger Federer en route to the Australian Open semi-finals, was twice broken in the first set but raced through the tie-break.
The world number 12 won eight of the last 10 games to triumph in 69 minutes.
Stefanos Tsitsipas will face Mikhail Kukushkin for the Marseille crown after the Kazakh ended the resistance of home favourite Ugo Humbert 6-4, 6-4.
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