I thought about something like that:
1. It could be played inside the big rectangle like a tennis game, yes.
2. when the ball hits you, the game freeze, and there is this mini-game right below the rectangle:
---- 1. a white thingy moves quickly from the left to the right following the horizontal line at the bottom of the screen.
---- 2. you have to push the righ button (A=normal hit B=smash C=drop shot, LEFT=...) at the moment when this big white thingy hits the smaller thingy.
---- 3. if you failed to push a button at the right moment, you missed the ball.
---- 4. if you succeed, the mini-game stops, and the normal game continue.
The letters under the tennis ball could list the different kind of actions available for each players.
... BUT i have no idea if it is a fun gameplay ; these are just random thoughts