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 today’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 slow down, relinquish momentum, think too much about what’s 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.

Swiatek, who won the 2020 French Open, is on a 34-match winning streak after eliminating No. 20 Daria Kasatkina 6-2, 6-1 today.

“I’m in a mindset now like: ‘It doesn’t matter.’ I mean, I’m going to be happy, regardless. My parents are going to love me, regardless. So I’m just going to go into it like another match,” the 18th-seeded Gauff said. “I mean, yeah, it’s a Grand Slam final, but there are so many things going on in the world right now, and especially in the U.S. a lot of stuff is happening right now, so I think it’s not important to stress over a tennis match.”

And then, in a message referring to the recent series of mass shootings in her home country, Gauff wrote in marker on a courtside TV camera: “Peace. End gun violence.”

A willingness to speak out about issues of significance, and a wide-lens view of the world, reflect the sort of maturity that has served her well, especially with so much attention from such a young age. She won the French Open junior title at 14. She became the youngest qualifier in Wimbledon history and then beat Venus Williams on the way to the fourth round there at 15.

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.

“I’m a little bit in shock right now,” Gauff said with a chuckle after using her still-improving forehand to open up the court and her long-terrific backhand to seal points against Trevisan. “I didn’t know how to react at the end of the match. I have no words to describe how I feel.”

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, in Swiatek, comes someone who will present a much tougher test and will require a higher level of quality.

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.

With only faint wisps of clouds overhead and the temperature hovering around 75 degrees Fahrenheit (about 25 Celsius), the two women’s semifinals followed a similar pattern.

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.

Gauff won six of Trevisan’s service games, raising her tournament total to 35 breaks in six matches.

Trevisan is a 28-year-old left-hander who entered the day on a 10-match winning streak, including her first career WTA title at Rabat, Morocco, a week before play began at Roland Garros. She also defeated Gauff the only previous time they played — in the second round in Paris two years ago.

But Gauff is nothing if not a learner and a striver, someone who looks better and better with a racket in her hand each time she takes the court.

A big part of that, perhaps bigger than her speedy serves that reached 115 mph (185 kph) today, is her preternatural cool, an old soul’s ability to see an obstacle and not be daunted.

Early on, a Trevisan shot landed near a baseline and was ruled in to make it 30-all. Gauff asked chair umpire Marijana Veljovic to come get a closer look, and the pair engaged in an extended discussion, both pointing at the landing spot. Gauff crouched down to inspect as some spectators whistled and jeered, as they often do here when players argue a ruling.

A smattering of boos then cascaded on Gauff when she walked to the net to check a mark after one of her backhands landed wide during a game in which Trevisan would break back to make it 3-all.

Unbothered, Gauff broke right back to lead 4-3, smacking a backhand winner that brought Mom and Dad out of their seats in the player guest box. The match would continue for about 50 more minutes, but the outcome would never again be in any real doubt.

“Honestly I wasn’t nervous going in today. I haven’t been nervous at all, which is a surprise,” Gauff said. “The only time I get a little nervous is maybe in the morning. I go for a walk in the morning so that kind of clears my head and after that, I feel great.”