Really cool and polished game! There is almost nothing wrong with it, only that the AI is just playing against themselves. Maybe the court could be a bit bigger, as it felt a little restricted, there is a lot of bumping into each other. The music is great and the sound effects when you score/lose a point is very fun. I like that the background is also not just a static image, the parallax effect is a nice touch. The AI is a bit exploitable sometimes, but overall really impressed by it.
The upgrades are not always very clear in what they do, maybe you could add a visual indicator that shows what changes. Really good game!