NEW YORK — With her newly familiar face rich in the calm that makes it look wiser than her ludicrous age and rather at peace amid tennis fracas, 15-year-old Coco Gauff spent the early evening Tuesday piling more brickwork on her growing, glowing reputation. In the first U.S. Open main-draw match of her life after her introductory surge last month at Wimbledon, she located her steadiness and let that carry her across some rapids.

She bested a fellow former junior No. 1 player and a fellow teenager, 18-year-old Anastasia Potapova of Russia, in a 3-6, 6-2, 6-4 winding road inside Louis Armstrong Stadium, the U.S. Open’s second-biggest court. An end-of-session crowd both enthusiastic and sparser than it should have been — half-full or half-empty — witnessed ­somebody born in March 2004 playing a U.S. Open third set in which she hit eight winners to her opponent’s two.

Off went the Floridian from Delray Beach to the second round Thursday, without even realizing she would have a day off before playing 112th-ranked Hungarian Timea Babos until on-court interviewer Pam Shriver reminded Gauff of this Grand Slam fact of life. “I’m still used to playing juniors; I forgot about the day off,” Gauff said with a smile.

“I mean, it was crazy,” she said in the interview room later. “I mean, obviously, I was nervous going out on the court, and I wanted to do well in my home Slam.”

She made those truths about juniors and nerves seem implausible with a comeback against Potapova, who beat three-time Grand Slam winner Angelique Kerber at the French Open, who ranks No. 72 to Gauff’s No. 140 and whose early heat dulled Gauff’s home-court advantage. Potapova went up and Gauff’s serve went awry, with four double faults in the first set, including three in an opening service game that qualified as dreary.

“After the first set, I had to kind of reset and just stop thinking about what will happen after the match and think about what happens in the match,” she said, and she did.

Her expression stayed intent and polished. She overcame Potapova as well as the U.S. Open’s renowned capacity for clamor, from the murmurs of the crowds at food stands to the screeching of the subway wheels to the whistle of the Long Island Rail Road. Her steadiness gathered as her opponent’s gave way. Gauff began to play steady points of moderate pace, then fantastic points of every pace as she claimed 10 of 12 games while she turned a 3-6, 0-1 deficit into a 3-6, 6-2, 4-1 lead.

“Coco! Coco!” the crowd yelled as Potapova’s winners often got claps countable on one hand. “That was my first match where people had a chant for me,” Gauff said, “so that was pretty cool.”

From there, Gauff overcame still more. That included the medical timeout Potapova took at 4-1 in the third set for attention to her shoulder. It included Potapova’s rebound from love-30 through long, solid rallies to hold for 4-2, and Gauff’s harrowing service game croaking with three double faults for 4-3. By the time Potapova worked through two break points and held for 4-4, Gauff had to reach into her stash of beyond-her-years moxie.

The stash does seem abundant within the daughter of a former college basketball player at Georgia State (her father, Corey) and a former track athlete at Florida State (her mother, Candi).

“Honestly, I just wanted to win so bad, I was trying to really dig deep,” Gauff said. “I was like: ‘You got to make her play. She’s not going to hand it to you.’ We had a couple long rallies, then I got some winners here and there. Honestly, I mean, I really don’t remember the match too well because everything is still a blur. But I do remember, I think, that 4-all game; it was just like a lot of battling.”

Much as she held off No. 60 Polona Hercog, 3-6, 7-6 (9-7), 7-5, in the third round at Wimbledon, Gauff went from 4-4 to happy here. She held serve with the help of a 102-mph service winner at 30-all that went into the corner and wreaked a harmless popup return. Then, even though Gauff would convert only seven of 19 break points to Potapova’s 5-for-5 effort, she had one ready for the ending.

It went to 15-40 with two Potapova errors that looked plenty forced and one Gauff cross-court forehand that looked plenty excellent and yielded a netted forehand. Then Potapova’s last shot whirred just long and left Gauff looking down at that and then looking up directly at her parents.

“I think I gave them a heart attack,” she said, and she ran up to a handshake between a 15-year-old and an 18-year-old who complimented each other profusely by Gauff’s account and hoped the future would bring further meetings.

“But hopefully in the finals, not in the first round,” Gauff said, and it did seem realistic.