She’s the French Open’s grizzled veteran—at age 21

Somewhere between the hotel, the locker room, and the red clay of Roland-Garros, she’d simply forgotten to put them in her bag—less than ideal when you’re trying to win a second career Grand Slam tournament.

Gauff was able to laugh it off and, once her coaches delivered her equipment, won her match anyway. But that brief oversight was a rare reminder of just how young she is.

Most of the time, her consistency, experience, and status as the world’s highest-paid female athlete make it easy to forget that ever since she burst onto the scene at 15, her career has been on fast-forward.

“I definitely feel like I’m…more mature than maybe some of my peers," said Gauff, now 21. “Playing tennis forces you to grow up faster for some people."

Especially when you break through as early as she did. Gauff made a stunning run to the fourth round of Wimbledon as a teenager in 2019 and has been a top-10 player for almost four years. In 2023, she claimed her first major by winning the U.S. Open. And on Saturday, Gauff will take her shot at a second when she faces world No. 1 Aryna Sabalenka in the French Open final. She got there by dispatching France’s Loïs Boisson, the Cinderella of this tournament, in just 1 hour and 9 minutes.

“She was just too good," said Boisson, who began the tournament ranked No. 361. “I just feel like I was running everywhere on the court today."

Gauff was always going to be a heavy favorite in that match. And despite the raucous home crowd under the roof of Philippe-Chatrier Court, she remained unflappable. Boisson found her so frustrating to play against that when her coaches whispered a piece of advice in the second set, she shouted back in French, “But I can’t!"

Even so, this week has irrevocably changed Boisson’s career. The 22-year-old will climb 296 places in the world rankings, to No. 65, and more than quadrupled her career earnings by taking home $790,000 in prize money for reaching the semis.

Gauff, on the other hand, has long been at the other end of the tennis food chain. Since turning pro, she has already racked up $24 million in prize money, and brings in at least that much annually through endorsements. What makes it all the more remarkable is that through all of it, her game has remained a work in progress. Gauff’s tennis education has unfolded in plain sight of the world.

Her serve, for instance, remains maddeningly inconsistent—she scores the lowest percentage of points behind her first serve of any woman in the top five. But her court coverage and her return game mean that she can keep herself in any rally long enough to do damage.

So it makes sense that the slower surface of Roland-Garros, which rewards baseliners and defenders, has turned into her happiest hunting ground. Gauff acclimated to clay early in her career by training in the South of France and has reached at least a quarterfinal here every year since 2021, plus a final in 2022. In fact, she is the youngest player to reach at least 25 match victories at a single major event since Maria Sharapova was dazzling the Australian Open in the late 2000s.

“My first final here I was super nervous and I wrote myself off," Gauff said. “This time I have a lot more confidence."

Sabalenka’s affection for clay is more recent. Better known as the best hard-court player in women’s tennis, she had never gone beyond a semifinal here in eight years of trying. But with a few tweaks to her free-swinging game, mixing in more variety, Sabalenka was able to take down the three-time defending champion, Iga Swiatek, 7-6(1), 4-6, 6-0 in her semifinal. Swiatek hadn’t lost a single match at Roland-Garros since 2021.

“It felt like a final, but I know that the job is not done yet," Sabalenka said. “I have to bring my best tennis and I have to work for that title, especially if it’s going to be Coco."

Write to Joshua Robinson at Joshua.Robinson@wsj.com