Who is Gavin MacMillan? All about Coco Gauff’s New Serving Coach

When Coco Gauff walked onto Arthur Ashe Stadium for her first-round match at the 2025 U.S. Open, all eyes weren’t just on the defending champion—they were also on her new service motion. Behind the change is Gavin MacMillan, a biomechanics expert known for transforming athletes’ careers.

The 20-year-old American tennis star admitted she wished she had more than six days with MacMillan before debuting the adjustments in one of the sport’s biggest arenas. Still, despite 10 double faults and plenty of tense moments, Gauff managed to grind out a three-set win over Ajla Tomljanovic, proving the new partnership might already be paying dividends.

Who is Gavin MacMillan?

Gavin MacMillan is a biomechanics specialist and performance coach who has worked with athletes across multiple sports, including tennis, baseball, and golf. His expertise lies in using video analysis, data tracking, and body mechanics to fine-tune performance, particularly in areas as technical and repetitive as the tennis serve.

In the tennis world, MacMillan gained international attention in 2022 when he helped Aryna Sabalenka reinvent her serve. At the time, the Belarusian star was struggling with double faults, sometimes hitting more than 20 in a single match. After working with MacMillan, Sabalenka turned her serve into a weapon, went on to win three Grand Slam titles, and rose to World No. 1.

Now, he’s applying similar methods to Coco Gauff’s game.

Why Did Gauff Turn to MacMillan?

Gauff, despite her speed, power, and all-court ability, has long struggled with her serve consistency. She led the WTA Tour in double faults this season, surpassing 300, and in 2024 had matches where she hit more than 20 in a single outing.

“I know that this is the part of my game that needs to improve if I want to get the results that I want,” Gauff said at the U.S. Open.

MacMillan’s goal is to stabilize her serve through mechanical changes, ensuring she has a repeatable motion under pressure.

What Changes is Gauff Making?

While Gauff hasn’t revealed every detail, she acknowledged she is now:

• Rotating her body differently during the service motion.

• Tossing the ball to a new location.

• Practicing until “my shoulder was hurting,” highlighting the intensity of the adjustments.

The results are mixed but promising. Against Tomljanovic, she sometimes reverted to “old habits” under pressure, but when she executed the new motion properly, it produced cleaner, more reliable serves.

Serving Slower—On Purpose

Fans may have noticed Gauff serving slower than usual in her first-round match. Her average first-serve speed progressed from 88 mph in the first set to 101 mph in the third, with her fastest serve clocking 117 mph.

According to Gauff, this was a deliberate choice: “It’s a personal choice to serve slower. I know if I need my serve to get up, I can get it up, but it’s about making sure I stay in the motion that we worked on.”

This strategy reflects MacMillan’s philosophy: prioritize form and repeatability over raw power—at least until the technique is second nature.

Why This Partnership Matters

For Gauff, teaming up with Gavin MacMillan could be career-defining. She is already a Grand Slam champion and World No. 3, but a consistent, powerful serve would turn her into an even more dominant force on tour.

MacMillan’s track record with Sabalenka suggests that technical breakthroughs can spark major success. If Gauff follows a similar trajectory, she could add to her Grand Slam tally and strengthen her bid for the World No. 1 ranking.

Gavin MacMillan is more than just a “serve doctor.” He’s a biomechanics innovator with a proven history of transforming elite athletes under pressure. For Coco Gauff, his arrival marks a fresh chapter—one where her serve may finally become the consistent, reliable weapon she needs to take her game to the next level.

As the U.S. Open progresses, all eyes will be on whether the Gauff-MacMillan partnership can deliver immediate results, or if this is the start of a long-term transformation.