At the US Open, men play with the extra-felt duty ball whereas women play with the regular-felt ball. The extra felt on the ball makes a huge difference in speed and the kind of power that players can generate from the ball.
Both the extra-felt and regular-felt duty balls are made by Wilson. But the idea behind two different balls for men and women is that the tournament wants to speed up the women’s game and slow down the men’s game.
Female players such as Iga Swiatek and Ashleigh Barty (when she was active on the tour) have not been fans of the regular-felt ball. Barty’s coach once said that she will never win the US Open because of these balls.
Tournaments before the US Open in Cincinnati and Canada also use different balls for men and women. Despite the criticism, tournaments have continued using different balls for many years.
Mixed doubles matches, which feature both men and women, use regular-felt balls. As a result, male players who play mixed doubles have to play with both regular-felt and extra-felt duty balls at the US Open if they participate in other events.
Occasionally, the balls get mixed up. Players such as Andy Murray, Andy Roddick, and Madison Keys have complained to the umpire during matches that wrong balls were mixed in.