I feel that this is cropped too tightly, try standing further back to get the whole of the man in frame. Also I think capturing the player as he throws the ball in to the air and moves the racket to strike it, would be a more pleasing pose.
A wonderful demonstration of when a person is too old and out of shape to server normally, adn has to alter his serve to compensate. I like the composition and feeling of motion.
You selected an excelent topic for your submission: the moving tennis racket works really good. To me the image looks a little grainy though and not crisp on the parts that should be well focused. Good luck.