NEW YORK — Things were not going well for Coco Gauff at the U.S. Open on Friday night.

Her shots were off. She was struggling to hold serve. She dropped a set. All the while, Gauff was talking to her coaches, Brad Gilbert and Pere Riba. Talking to herself. Slapping her thigh or putting her palm to her face.

And when the No. 6-seeded Gauff needed to lift her game, when she needed to get headed in the right direction before it was too late, she did just that. Did she ever. Asking for more noise from the partisan fans at Arthur Ashe stadium — and, naturally, getting it — Gauff grabbed the last 10 games for a 3-6, 6-3, 6-0 victory over No. 32 Elise Mertens to reach the fourth round at Flushing Meadows.

"The energy today definitely helped me. I felt you guys," said Gauff, a 19-year-old from Florida. "I played every point my hardest."

The 2022 French Open runner-up won for the 14th time in her past 15 matches, all on hard courts.

That stretch follows a first-round exit at Wimbledon in July and includes the two biggest titles of Gauff's career.

This was also her second win this week after falling behind by a set — and the ninth time she has produced that sort of a turnaround in a Grand Slam match.

"The three-setters show everybody else that I'm not going down without a fight," said Gauff, who made it to the quarterfinals in New York a year ago.

To return to that round, she will need to beat Caroline Wozniacki, a 33-year-old who is playing in her third event since coming out of retirement. Wozniacki won the 2018 Australian Open and twice was the runner-up at the U.S. Open before walking away from tennis 3 1/2 years ago to start a family.

For Gauff, the beginning of the second set was key.

After Mertens, a 27-year-old Belgian who reached the final four at the Australian Open five years ago, held to go up 1-0, she held five break points in an 18-point Gauff service game. But Gauff held steady there. Then, with Mertens leading 3-2 in that set, Gauff really began to heed the advice of Gilbert, who could be heard repeating various phrases to her: "Be positive; be energetic here" or "Be physical now, every point" or "Just got to stay with her."

And Mertens provided an opening. She sailed a backhand long, then sent a rushed forehand wide to hand over a break and a 4-3 lead. Gauff turned to her guest box and punched the air, then motioned to the raucous fans to get louder.

Facing a break point in the next game, Gauff delivered a forehand that clipped the net tape and landed in for a winner. She puffed her cheeks and exhaled. She eventually held there to go up 5-3 in that set, which soon was hers.

The third set was never much of a contest.

Gauff kept playing cleaner, going from 23 unforced errors over the first two sets to just two in the last.

And Mertens faltered. After eight unforced errors in the first set, she made 31 the rest of the way.

"After I lost that first set," Gauff said, "I told myself: There's ... a lot of the match to play."