Keep your eyes OFF the ball...
I'm no tennis instructor, but I wished I'd known about this ten years ago:
I got hit in the eye by the ball after I miss-hit a backhand slice.
The ball missed all the bones in my face and got me square on the eyeball.
I was laying on the floor trying not to throw up for quite a while, and it took hours before I could see even blurryness out of it.
Turns out my retina detached along about a quarter of its edge (up near the front of the eye). Like a wall's worth of wallpaper starting to roll down if the skylight is your pupil, and the rug is where you focus.
Anyhow, I've had 2 lots of laser zapping over the years, and the retina on the focusing area is starting to wrinkle (the rug on the floor) so it's affecting my vision.
I have no guarantees I'll die with 2 eyes.
If you're not a genius tennis player who never miss hits it might be worth getting eye protection. I wish I had. I wear it for any sport at all now, even PING PONG! as you can see HERE
I found out that the guy who ran the table tennis club I went to knew someone who lost an eye the same way but with a PING PONG BALL! He miss-hit a forehand loop.
Be careful out there...