You can start and save your tennis activities, track distance, steps and heart rate on the tennis court, all directly from your Apple Watch. If your Apple Watch is running watchOS 3, you can also count your tennis swings using the Device Motion capabilities that are added to watchOS 3.
To review your tennis swings breakdown, you must purchase the Pro Edition.
If you have trouble recording your tennis activities using your Apple Watch, most likely it is because of connectivity issues. This step-by-step trouble shooting guide should help resolve your problems.