Ragdoll Tennis is one of those games that seems absolutely impossible when you first try it, so don't get discouraged at first. You control your floppy Pete Sampas with either [WASD] or the arrow keys. Your number one priority, of course, is to hit the ball with your square racket, but depending on your speed, spin, and other factors, you can make "super" or even "special" shots that are harder for the other side to block. Train up against a wall, and then enter tournaments. As you defeat opponents, you'll earn points to improve your character's stats and unlock customization of his colors. Word to the wise: max adrenaline first. It's by far the most important stat.
Although the game has a definite learning curve, if you stick with it, there's a lot of fun to be had with this one.
Thanks to plasmasheep for suggesting this!