Coco Gauff has used a powerful serve and great court coverage to stand out among the emerging young stars in women’s tennis. Dr. Mark Kovacs, a sports scientist and coach who has worked with Gauff, helps explain the parts of her game that make her so good, as well as aspects she can improve.

Coco Gauff reacting after her second-round victory Thursday over Timea Babos at Louis Armstrong Stadium. Uli Seit for The New York Times

A Great Serve in the Making

Gauff, at 15, is nearly two years younger than anyone else in the United States Open, yet she already serves harder than many of the women in the tournament, reaching 118 m.p.h. “She has all the makings of having possibly the best serve ever,” Kovacs says.

But Kovacs cautions that Gauff is still a work in progress, with lots of room to grow. Not great news for her competitors.

Below, Kovacs highlights the parts of Gauff’s serve that are similar to other great servers, and the places where she can improve.

Foot alignment allows her to quickly rotate her hips Foot alignment allows her to quickly rotate her hips Storing energy Storing energy Could rotate hips more Could rotate hips more Could rotate hips more Could rotate hips more Could rotate hips more Could rotate hips more Elbow Chest Racket Off the ground Elbow Chest Racket Off the ground Elbow is tucked Elbow is tucked Side where ball made contact Forearm pronates Shoulder rotates Forearm pronates Shoulder rotates Side where ball made contact













































Gauff begins with a relaxed, aligned and balanced posture, allowing her to efficiently store and release her energy. Keeping her left arm straight allows for a consistent ball toss. She pushes hard into the ground, using the muscles of the right leg to store energy in her lower body. Kovacs says she could be rotating her hips more, about 15-20 degrees clockwise during this loading stage. Here, she overcompensates for her lack of hip rotation by trying to get more range from her shoulder and elbow. Her right elbow shouldn’t be as visible from this angle. If she continues to rely too much on her upper body, it could lead to shoulder and arm problems. This is called the cocking stage, where she stores energy in her upper body. Kovacs says every great server looks like this. She has great shoulder external rotation, pointing her chest and elbow to the sky, and her racket straight to the ground. She has good upper-body acceleration, and is in a good shoulder-over-shoulder position. Her elbow is still going up toward the sky, her head is up and her eyes are on the ball. Many poor servers have already dropped their heads at this point. As she makes contact, her body moves forward, with her shoulders perpendicular to the net. Her left elbow is tucked into her side, a sign that she hasn't opened up too early. She follows through. She rotates her right shoulder inward and pronates her forearm. Her arm is straight, with the tip of the racket pointing down. These movements cause the side of the racket that made contact to point away from her. After landing, her right leg kicks up for balance so she can quickly regain her position for the next shot.

Mastering Motion

Gauff’s movement is probably her best asset right now and what gives her the ability to compete with top players. Kovacs says she moves efficiently and reminds him a lot of Sloane Stephens, whom he worked with at a similar age.

Kovacs says the players both cover ground in large steps, whereas most girls at that age cover them in small, less efficient steps. “She takes half a step less than a lot of other girls to cover the same distance.”

This makes her faster and more efficient, and places her in a better position to defend.

Sloane Stephens reaches to return a shot from Anna Kalinskaya during the first round of the U.S. Open. Adam Hunger/Associated Press

In her first-round match against Anastasia Potapova on Tuesday, Gauff repeatedly displayed her trademark speed and footwork, crisscrossing the court to run down balls. It was never more evident than during the final point of the match, shown below.