You're in a competition of endless tournaments. Win each tournament and face harder, more experienced tennis players. As you win you will obtain diamonds in order to purchase new characters.
To play all you must do is swipe in the direction you want the ball to go. Your character will do the rest, so all you need to do is keep you eye on the ball! It's much harder than it sounds, so be aware! Some opponents will be an even match for you... Or will they? Enjoy!