“Eighty-twenty, yeah,” CiCi Bellis said. Then she paused, and sent her eyes skyward, pondering; a smile broadened across her seemingly untroubled face. Bellis smiles more than most professional tennis players. “Or maybe seventy-thirty. Yeah, seventy-thirty.” She’d just finished a two-hour morning session on Practice Court 6 at the U.S. Open, and we were talking about how she practices. The ratio under discussion was practicing strengths to practicing weaknesses. “Yeah, there are coaches who really want to work a lot more on what needs work,” she said. “But that wouldn’t work for me. I mean, in a match, it’s about your strengths, right? And when you practice your strengths, you feel positive.” I mentioned that she had been smiling throughout the session. She smiled again. “I love to practice,” she said.

Catherine Cartin Bellis, who prefers CiCi, is eighteen years old, the youngest player among the top fifty on the women’s tour. (Her ranking, going into the Open, was No. 36.) The women’s game is a very different one—more powerful, breakneck, demanding—than it was in 1979, when Tracy Austin won the Open three months from her seventeenth birthday. The body you need to compete now takes years to develop. Bellis is listed at five feet seven inches, and a hundred and twenty pounds. As we talked in the so-called players garden, a walled-off patio in the shadow of Arthur Ashe Stadium, Maria Sharapova stood nearby, chatting with an agent. The physical difference was striking. Bellis said that she spends a lot of time in the gym, and that she enjoys that, too. The strengths that I watched her practice—her ground strokes, especially her forehand, and particularly her inside-out forehand—offered proof of what every good tennis coach believes to be true, even if evidence can be hard to find: power is not so much the result of size or even strength but of technique, timing, and racquet-head speed. Bellis crushes the ball, and that, along with her quickness, court sense, and cloud-free attitude, has made her a standout among, suddenly, a batch of promising American teens.

Bellis was born in San Francisco, and grew up south of the city, in the affluent suburbs of Hillsborough and Atherton. Her mother, Lori, was a serious junior player. Bellis took her first lesson at three, and played as a child at the exquisite Burlingame Country Club and on a court in the back yard of her Atherton home. She was coached as a child by Monique Javer, a pro who cracked the top hundred a couple of times in the late eighties and early nineties. Bellis knew before she was ten that she might be a successful tennis player, she told me, because she could sense how readily she picked things up. “And, I mean, I loved tennis, loved it!” she added. As a junior, she won five national championships, and she reached No. 1 in the girls’ eighteen-and-under rankings in the spring of 2014. She was fifteen.

Not long after, she won the U.S.T.A.’s under-eighteen girls’ national championship, which earned her a wild card into the U.S. Open. For her first-round match she drew the twelfth-seeded Dominika Cibulkova, of Slovakia, and beat her in three sets. That set off a forty-eight-hour CiCi fest on the Flushing Meadows grounds and in the media; Open-goers swarmed to her late-afternoon second-round match out on Court 17. She wound up losing that match, to the little-known Zarina Diyas, of Kazakhstan, in three sets, but she had established herself with New York tennis fans. They lined the walkway two deep behind her practice court on Saturday morning.

Bellis officially turned pro last summer, passing up a scholarship to Stanford. (She’d been home-schooled, like so many young tennis players, for years.) Last fall, with the U.S.T.A.’s new national tennis center, in Lake Nona, south of Orlando, about to open, she and her parents relocated to the area. Along with being America’s top-ranked teen, male or female, she is one of the top prospects—Frances Tiafoe, who took Roger Federer to five sets the other night before succumbing, is another—who is fully rooted in the U.S.T.A.’s development program. The coaches with her in Flushing Meadows, Anibal Aranda and Ola Malmqvist, are members of the association’s player-development team. Bellis has no private coach. When she’s not playing tournaments, she does her training at the national center. For years, the program failed to turn out top players, but Bellis has embraced the center, with its state-of-the-art courts equipped with PlaySight technology, which allows players and their coaches to review video and data of a training session in progress, instantly, at a courtside kiosk. “I’m a really visual learner, so it helps me—the video, the graphics,” Bellis told me then.

When I saw her last weekend, she was enthusing about a monthly meeting she has with a coach in Lake Nona devoted to mental strength. “It’s, like, to help you stay calm during matches,” she said.

Yogic breathing?

“Yeah!” she said, and laughed. “Yogic breathing! Spot on!”

Bellis’s first-round opponent was Nao Hibino, of Japan, a twenty-two-year-old who’d had most of her success on the women’s tour in doubles. She and Bellis had met twice before, most recently in Rabat, Morocco, at one of those small, far-flung events on the I.T.F. circuit, a rung below the W.T.A. Bellis won both those matches, though they’d been close. Their match at the Open, originally scheduled for Tuesday, was rained out, but it was brilliantly sunny, the sky blue-blue and high, when they walked out on Court 17 on Wednesday afternoon, Bellis looking relaxed under big headphones.

She continued to appear relaxed even as she was broken quickly, in Game 3, and even as her forehands sailed long and wide. Hibino was firing at a very high level, as even a player ranked seventy-seventh can do on the right day, making few errors and smacking penetrating forehands, alternately struck flat and shaped with heavy topspin, which seemed to throw off Bellis’s timing. Bellis was broken a second time, and lost the set, 3–6.

When Bellis broke Hibino in Game 1 of the second set, she seemed to loosen up, too, smiling at her coaches and dancing a bit in place before she served. She took the set 6–4, then sat courtside, studying her data on the scoreboard. She got the attention of her coaches, seated across the court from her, gave her head an exaggerated shake, spread her arms, and laughed: crazy match.

There were few empty seats as Set 3 got under way. Bellis held at love in Game 2, then broke at love in Game 3. Serving for the match up 5–4, she hit a couple of errant forehands, then double-faulted, then hit another poor forehand: broken. Bellis would not shake that game, and would not win another. A gruelling loss: 3–6, 6–4, 5–7. Her weakness, her second serve, had shown glaringly: four double-faults. Still, it was a matter of her opponent winning a half dozen more points over the course of a couple hours. Tennis.

Bellis left the court quickly, with no wave to the crowd. She didn’t speak with the media afterward; there were no media requests to speak with her. The tournament was moving on, and after playing some doubles here, she would be doing the same, heading back to Lake Nona to do what players, especially teen-age players, have to do: get better.