NEW YORK — After serving poorly for one set during a US Open victory, Coco Gauff headed to a practice court for some extra work on that stroke Wednesday night.

She realized her tosses were too far in front of her and too low, so even if she beat 99th-ranked Tatjana Maria 6-4, 6-0 to reach the third round, Gauff wanted to fix the issue.

“I didn’t serve great at all,” Gauff said after stretching her winning streak to nine matches at the site of her first Grand Slam title a year ago. “I got lucky. I think I only got broken once, but probably could have — should have — gotten broken a couple of more times.”

Gauff, a 20-year-old from Florida, had all sorts of trouble at the outset under the lights at a hot and muggy Arthur Ashe Stadium against Maria, a 37-year-old from Germany who made it to the Wimbledon semifinals two years ago and was hitting slices on nearly every forehand and backhand.

“I love Tatjana … [but] she’s very annoying to play,” Gauff said. “Tonight was just an annoying match. The balls, you were able to hit winners on them, but because she’s slicing it, you have to generate all the power. Sometimes it can be more physically tiring than it looks, because you have to generate literally 100% of the power every single time, compared to maybe somebody where you can just redirect.”

There were seven double-faults for Gauff in the initial set. She put in fewer than half of her first serves. She faced five break points, saving four. Problems arose in other aspects of her play, too, including winning the point on just 9 of 17 trips to the net, and a total of twice as many unforced errors, 20, as winners, 10.

Her body language was frequently negative as can be after some of the mistakes. Gauff would look at her guest box and put her palms up as if to say, “What is going on?” She would roll her eyes or put a hand on a hip, including after one wild swinging volley that sailed several feet long.

When the No. 3-seeded Gauff served for the opening set, she double-faulted twice and faced a pair of break points that would have allowed Maria to get the score to 5-all. But Maria missed a forehand on the first, and Gauff struck a cross-court forehand passing shot on the second, then finished off the game.

The second set was a completely different story.

Gauff put together 15 winners to merely five unforced errors — two of which were double-faults — and went 10 for 11 at the net.

“She’s a mentally really strong player,” said Maria, whose two daughters, ages 10 and 3, sat in the stands. “She has a lot of power. I knew that I had my chances today, because I know that my game style is not one she prefers. I had my chances in the first set that I didn’t take. This kind of player … if you don’t take your chances, then it goes away. This is what happened today.”

On Friday, Gauff will take on 27th-seeded Elina Svitolina, a three-time major semifinalist who was a 6-1, 6-2 winner against Anhelina Kalinina, for a berth in the fourth round.

Gauff arrived in New York off a series of early exits elsewhere. That included two consecutive losses at hard-court tuneup events this month, which followed eliminations in the fourth round at Wimbledon and the third round of the Paris Olympics, both in July.

“It’s been a tough few weeks,” Gauff said.

Gauff’s nine double faults Wednesday were three more than she had in her opening-round match against Varvara Gracheva. She put only 43% of her first serves in play Wednesday, way down from the 64% mark in her opening match.

Despite the serve troubles, Gauff closed out both her first- and second-round matches with a 6-0 set. She is the first defending US Open women’s champion to win two bagel sets in her first two matches since Serena Williams in 2013 (won title).

Gauff also improved to 22-3 in majors since the start of her 2023 US Open title run. That’s the most major wins by any woman in that span.

ESPN Stats & Information and The Associated Press contributed to this report.