PARIS — Coco Gauff sat plain-faced in her French Open changeover chair after moving one set away from her first Grand Slam final at 18 — just 18! — and calmly snacked on some cut-up pieces of pineapple and other fruit, seemingly without a care in the world.

Her opponent in Thursday’s semifinals at Roland Garros, Martina Trevisan, was taking a medical timeout so her right thigh could be treated and taped by a trainer. It was the sort of delay that might rattle some players, might make them cool off, relinquish momentum, think too much about what was at stake.

Not Gauff. Not on this sunny, breezy afternoon at Court Philippe Chatrier. When the American arrived in Paris, she celebrated her recent high school graduation by posing for cap-and-gown photos near the Eiffel Tower while holding her diploma. After wrapping up a 6-3, 6-1 victory over the 59th-ranked Trevisan, Gauff will be back in that stadium on Saturday to face No. 1 Iga Swiatek for the championship.

The 18th-seeded Gauff, who turned 18 in March, is the youngest finalist at the clay-court major tournament since 2001, when Kim Clijsters was the runner-up a day after her 18th birthday.

Gauff has won all 12 sets she has played over the past two weeks. She did not need to navigate the most arduous path to get this far: Because of all manner of surprising results and early exits by top players from the bracket, she has faced only one seeded foe so far, No. 31 Elise Mertens.

Now comes someone who will present a much tougher test and will require a higher level of quality. Swiatek is not only already a Grand Slam champion — she won the French Open in 2020 — but is unbeaten in her past 34 matches after eliminating No. 20 Daria Kasatkina 6-2, 6-1 on Thursday.

The men’s semifinals are Friday, with 13-time French Open champion Rafael Nadal playing No. 3 seed Alexander Zverev, and 2014 U.S. Open champion Marin Cilic facing No. 8 Casper Ruud.

Swiatek has won her past five tournaments, last losing in February, against 2017 French Open champion Jelena Ostapenko. The only Polish player to win a major singles title in tennis is now on a winning streak that equals one Serena Williams had in 2013. The only longer run this century was a 35-match unbeaten stretch by Venus Williams in 2000.

Swiatek and Kasatkina were even at 2-all. But Swiatek claimed 10 of the last 11 games.

Gauff and Trevisan were even at 3-all. But Gauff claimed nine of the last 10 games.

The Associated Press contributed to this story.