Coco Gauff fired her primary coach, and that decision is about to come under scrutiny at the US Open tennis tournament.

Her new coach, Gavin MacMillan, is a biomechanics specialist Gauff is hoping can help her fix ongoing struggles with her serve.

So far, good enough.

The third-seeded Gauff, the American star, outlasted the formidable Ajla Tomljanovic 6-4, 6-7 (7), 7-5 — despite a flawed serve that proved solid enough.

During an on-court interview after the match, Gauff said preparing for the US Open for the past 5-6 days with a new coach “has been really tough and mentally exhausting. … I think I’m just trying to improve with each match.’’ 

Both players had trouble holding serve, but neither had trouble powering the ball from the baseline. They spent much of the match slugging it out during long rallies. They combined for 115 unforced errors, but they also cracked 41 winners — 29 from Gauff.

Although Gauff had 10 double faults, she also uncorked four aces and her fastest serves topped 110 mph.

And in the end, it was pure Gauff. She struck a signature backhand for a winner and match point.

Gauff called it a tough match and it lasted just three minutes shy of three hours.

“Ajla was tough,” Gauff said. ‘I felt she was getting so many balls back and I was trying to push her back and she was standing on top of the baseline.

‘But, yeah, it wasn’t the best but I’m happy to get through to the next round.’’

Coco Gauff vs. Ajla Tomljanovic highlights

Coco Gauff interview

Coco Gauff vs Ajla Tomljanović: US Open score

Coco Gauff def. Ajla Tomljanovic 6-4, 6-7 (7), 7-5 in first round of US Open.

Coco Gauff next opponent

Coco Gauff is slated to face Donna Vekic in the second round on Thursday, Aug. 28.

Uh oh, Coco

Gauff led 4-2 in the second set before Tomljanovic fought back and went up 5-4. Soon Gauff looked clutch, saving two set points before knotting the set at 5-5. But the set went to a tiebreak and Tomljanovic was in control the whole way before closing it out 7-2.

Coco Gauff bounces back after shaky start

Gauff dropped the first two games of the match and she looked especially apprehensive with her service. But she fought back. Showing increased confidence, she won six of the next eight games and secured the set, 6-4.

How to watch Coco Gauff at US Open Round 1

Match time: 7 p.m. ET

TV: ESPN

Streaming: ESPN+, Fubo

Opponent: Ajla Tomljanović of Australia. Gauff and Tomljanović have played only once, with Gauff winning that match at the Paris Olympics 6-3, 6-0.

Watch the US Open with ESPN+

Coco Gauff: What to know

Gauff, who won the US Open in 2023, is seeded third. She recently parted ways with her primary coach, Matt Daly, and hired biomechanics specialist Gavin MacMillan as she continues to struggle with her serve. Gauff and her first-round opponent, Ajla Tomljanović of Australia, have played only once, with Gauff winning that match at the Paris Olympics 6-3, 6-0.