Top-seeded Roger Federer had to work for it, but he ultimately beat Frenchman Gilles Simon on Friday to advance to the semifinals at the Swiss Indoors Basel in Switzerland.
The 37-year-old world number three, who first won the Swiss title in 2006, was a break down in each set to Romanian Copil, seeking his first title.
It's the second final of the year for Copil.
"I'm starting to really get to understand the conditions and it was consistent, my most consistent match by far".
He will be the third seed in Paris and has a bye into round two, where he will face Croatia's world number six Marin Cilic. "I'm always happy when I win, I'm happy I fought through like this", Federer said.
"I was feeling good on court apart from closing it out, I had a few errors there".
Federer, an eight-time victor of his hometown event, was far from his best but managed to come back from a break down to win the first set in a tiebreaker on Friday.
Seventh-seeded Daniil Medvedev, who is also in Federer's half of the draw, beat Maximilian Marterer of Germany 6-3, 7-5.
Federer, 37, spent nearly 90 minutes less on court Saturday than Copil, who has had to come through six matches to reach his second career ATP final. He needed 44 minutes to claim the opening set but was brought back to level-pegging as Zverev won the second in a tiebreaker. Federer has never lost a championship match to an opponent ranked below World Number 87.
Fifth-seeded Kei Nishikori of Japan routed local favorite and top-seeded Dominic Thiem in the quarterfinals at Vienna, Austria, dispatching him 6-3, 6-1 in just 66 minutes.
The 32nd-ranked Simon continued to make the match a struggle, with the pair trading four breaks in a row until Federer took a 4-3 lead.
Copil again showed his steel with an early break in the second set before Federer made another fightback with an overhead victor to break back in the seventh game.
"I came through qualifying and I didn't know what to expect but here I am", said the 28-year-old Copil.
Federer put the pressure on Simon's serve after battling back from 15-40 down in the 10th game and Simon crumbled with two straight errors.