The 7 have to come from this selection:
Which exercises can I track in the Exercise app on my Fitbit device?
Note it also says, "If your exercise isn’t included, choose Workout."
But I think tennis would be fine. Not every one agrees, but my thinking is that the category chosen is mostly for your own reference, and to determine which statistics are shown in workout summary. It will figure out the intensity, and calories burned, from your heart rate.