You can start and save your tennis activities, track distance, steps and heart rate on the tennis court, all directly from your Apple Watch.
The new Device Motion framework introduced in watchOS 3 allows you to count your tennis swings
based on your arm swinging motion that can be detected by the Apple Watch built-in sensors.
To review your tennis workout metrics, you must upgrade to 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.
If you like to get a better understanding on how the Apple Watch sensors work, please check out these blog articles.