World number five Stefanos Tsitsipas came from a set down to beat Lorenzo Musetti 4-6, 6-3, 6-0 on Saturday and reach the final of the ATP event in Lyon.
In Sunday's final, the 22-year-old Greek will face Britain's Cameron Norrie who raced past Karen Khachanov 6-1, 6-1 in the other semi-final.
Norrie, 25, had already played on Saturday morning having beaten French hope Arthur Rinderknech 6-3, 3-6, 6-3 in a quarter-final suspended overnight with the last set still to play.
Musetti, 19, came into the event as an alternate but made the most of his chance with wins over seventh seed Felix Auger-Aliassime, Sebastian Korda and Aljaz Bedene.
The Italian teenager grabbed the first set but Tsitsipas gradually took charge.
"It was important I had a match like this," said Tsitsipas on court after the match.
"It makes me stay humble and focus on some detailed things I can potentially improve and get better at on clay.
"I had to stay calm. It was difficult to be a set down and have to kind of refresh myself and come anew in the second set.
"He was still putting in a lot of work and making me work for every single point. It wasn't easy."
Tsitsipas is seeking his second title of the year after winning the Monte Carlo Masters. He also reached the final in Barcelona where he lost to Rafael Nadal having had a match point.
British number two Norrie, ranked 49 in the world, will be looking for his first ever title in his first meeting with Tsitsipas.