Playing tennis like a ninja requires good movement.
Good movement is made of two things: speed and grace.
Grace is about moving smoothly. A ninja’s every move is coordinated. Every movement is necessary. There is no wasted energy.
Are there any extra movements when you swing? What about when you run? What about between points?
Practice your swing in slow motion and pay attention to the flow of movements in detail. Can you stay balanced the whole time? Your arm should move like a ribbon flowing through air. Your racket an extension of your arm. Then you are graceful.