One option could be to use the Other workout in the Workout app on Apple Watch. This is generally acceptable for workouts that are not already available in the Workout app, but you can also label the workout as tennis. This excerpt from the Apple Watch User Guide explains how to label a workout —
Start a workout with the type Other. When you’re done, swipe right, then tap End. Tap Name Workout, tap the type of workout you’ve finished—tennis or Tai Chi, for example—scroll to the bottom of the workout summary, then tap Done.
You can also use an app called TennisKeeper, which offers an Apple Watch app as well. Though I have never used this app, its page on the App Store advertises Apple Watch integration —
This app reads your fitness data and writes your tennis activity as a Workout to Health. The health data is correlated to your tennis activities.