The Prone (face and chest facing the floor and on all fours) Plank is a great exercise by itself; when you add a static leg raise in the equation, can be one of the toughest core exercises you will do.
The exercise is very simple, but very, very effective. Squeeze your core while you hold the position and feel your whole body working.
Start with 15 secs and try to increase your time from there. If you can get up to 75 secs, you will be right there with the Tennis Pros!
Pauly P