American tennis star Coco Gauff has parted ways with one of her coaches, Matt Daly, and brought on biomechanics expert Gavin MacMillan, according to multiple reports.

Gauff, 21, who has struggled with her serve since her first-round defeat at Wimbledon last month, made the coaching shift days before the start of the U.S. Open.

“Only have good things to say about Coco, enjoyed working with her,” Daly told Bounces in confirming the change.

MacMillan previously worked with current world No. 1 Aryna Sabalenka to fix her serve and founded the Sport Science Lab — a company that specializes in sports performance, physical rehabilitation and general fitness.

MacMillan was on the court as Gauff was hitting with Ukraine’s Elina Svitolina at Arthur Ashe Stadium on Wednesday morning.

Initial reports indicated that Gauff also had fired longtime coach Jean-Christophe Faurel. However, ESPN noted that Faurel was on the court with MacMillan and Gauff on Wednesday.

“A lot of things add up, and this could be a really good relationship,” ESPN’s Darren Cahill said of MacMillan. “He’s very thorough. … I’ve spoken to him many, many times before, and the fact that he’s got experience with different athletes from different sports, both male and female, he uses the new technology, he goes to the video a lot, really breaks down the biomechanics.”

Gauff’s coaching shake-up comes after she has struggled with her serve most of the year. She is coming off a Cincinnati Open quarterfinal loss to Italy’s Jasmine Paolini in straight sets, in which the two-time Grand Slam winner hit 62 unforced errors.

Last year, Gauff fired former coach Brad Gilbert following her fourth-round exit at the U.S. Open, where she failed to defend her 2023 title.

Gauff won the French Open in May.