Like others I think you should get her tickets to the Indian Wells tournament in March (assuming you’re American). Make a 4 day trip of it or so, it’s known by fans and players alike for being a tennis paradise because of the facilities as well as the surrounding environment.

Choosing “things” like rackets, apparel, shoes etc is kind of risky because people can be very particular and like to pick that out themselves but a gift card to Tennis Warehouse or something would be too impersonal.

Also studies have shown that people usually say they look back and are happier with the money they spent on “experiences,” vs “things” so if you went with her I think it could be a memorable you both always look back on fondly.