Coco Gauff wins the third set and advances to the second round of the US Open. (0:30)

NEW YORK -- Here's how new all of this is to Coco Gauff: She didn't realize she has to play only every other day at the US Open.

"I'm still used to playing juniors," the American said with a chuckle, "so I forgot about the day off."

She's still just 15. She's competing in her second Grand Slam tournament. Yet she's definitely showing that she can perform like someone much older and more experienced.

With her parents jumping out of their front-row seats over and over again and a raucous partisan crowd backing her at Louis Armstrong Stadium, Gauff trailed by a set and a break, then again by a break in the third set before coming up big down the stretch to get past Anastasia Potapova of Russia 3-6, 6-2, 6-4 in her debut at Flushing Meadows.

Gauff's victory came just hours before another American favorite -- No. 11 seed Sloane Stephens -- suffered a massive upset at the hands of Anna Kalinskaya, a 20-year-old Russian who is ranked 127th and had never won a main-draw match at a Grand Slam tournament.

"Honestly, I mean, I really don't remember the match too well," Gauff said, "because everything is still a blur."

Here is what is clear: Gauff displayed the same sort of gumption she did while saving match points in a Centre Court comeback at Wimbledon during her captivating run to the fourth round there last month. She simply does not give in or give up.

Gauff did not start well against the 72nd-ranked Potapova, falling behind 3-0 early. Gauff, who is based in Florida, double-faulted three times in her first service game on a breezy evening with the temperature sliding to about 70 degrees.

Soon enough it was 5-1, and then that set belonged to Potapova, with Gauff turning to her parents in her guest box and putting her palms up, as if to ask, "What can I do differently?"

"I was telling them to get hyped up more. I needed more positive energy. I was nervous. Just looking at them when they're giving me a fist pump gives me a little reassurance,'' Gauff said. "I think they were nervous. They wanted to stay more reserved, too. I was like, 'No, I need you guys to come on, stand up.'''

They obliged, yelling and clapping after what seemed to be each point Coco won. Dad also repeatedly pounded a fist on his chest.

"I think I gave them a heart attack, especially my mom,'' Gauff said. "And my dad, he looks a little bit tired over there, too."

Not much later, when Potapova broke to open the second set, Gauff's shoulders slumped, and she whacked her racket against her right leg. That's when her turnaround began. With spectators roaring their approval, Gauff broke right back and was on her way to dominating the second set.

There was some trouble still awaiting. Gauff went down 1-0 in the third set, then took four games in a row. But at 4-1, Potapova took a medical timeout, and a trainer massaged her right shoulder. She quickly pulled even at 4-all.

Gauff took command from there, breaking and then serving out the victory. This is still all so new to her. But she doesn't show it.

"I know we're going to see each other a lot in the future. I hope we do see each other in the future," Gauff said, "but hopefully in the finals, not in the first round."

Stephens, who won the US Open in 2017, lost 6-3, 6-4 to Kalinskaya, who became the lowest-ranked player Stephens has lost to in her major career.

Stephens' biggest problem was unforced errors; she finished with 33, more than double Kalinskaya's 15. Kalinskaya had been 0-5 at majors and 0-4 against top-20 opponents and had never appeared in a night match at a Slam.

"I was playing one good point, one bad point, one good point,'' said Stephens, who recently reunited with Kamau Murray, who coached her to the 2017 title at Flushing Meadows. "The inconsistency doesn't help me at all."

In other women's action Tuesday, Svetlana Kuznetsova, the 2004 champion, lost 7-5, 6-2 to American Kristie Ahn. Two-time finalist Victoria Azarenka also lost at night, falling 3-6, 6-3, 6-4 to No. 9 seed Aryna Sabalenka.

Earlier, defending champion and No. 1 seed Naomi Osaka fell behind against 84th-ranked Anna Blinkova and wasted a match point in the second set, but then she put together a 6-4, 6-7 (5), 6-2 victory.

The Associated Press contributed to this report.