The fourth-seeded Gauff, who won the New York Grand Slam in 2023, was broken only once to top the 53rd-ranked Sonmez 6-3, 6-4 in a hard-hitting battle fought mostly from the baseline.

"It's incredible to be back on this court," an exuberant Gauff told the on-court interviewer. "I love playing in this atmosphere."

Gauff, whose service woes last year contributed to a fourth-round exit, came into Flushing Meadows this time feeling much better about the stroke, having won the WTA 1000 event in Cincinnati this month.

The American star blazed through the opening set without facing a break point, with Sonmez often unable to return Gauff's serve or offering only a weak reply that could be batted off easily.

Gauff said she had worked to make the toss more consistent under the tutelage of biomechanics expert Gavin Macmillan, who previously helped Aryna Sabalenka retool her serve, improvements that helped the Belarusian rise to number one.

"It's been a gradual change with the serve," Gauff said. "Slowly it's adapting to maybe the way it should look."

Gauff broke early in the second set, but the Turkish player raised the level of her game with heavy ground strokes and sneaky forays to the net to finish points with deft volleys.

Sonmez broke back for 2-2 and briefly took the lead.

But at 2-3, Gauff fought off multiple break points in a lengthy game that the American eventually nabbed with another big serve.

Gauff called the hold for 3-3 as "real important."

"I like to challenge my opponent to let them keep up the level the whole match, and if they do, you know I clap my hands, and I walk off court with my head held high," Gauff said. "But I was able to settle down."