As with any area of your body, when it comes to your belly, you absolutely need to start with the basics and build fundamental and baseline strength before you move on. Far too often, I see people in the gym putting themselves in the position for serious injury by doing very advanced exercises (that they've probably seen on a video somewhere or have seen someone else doing) without having the necessary foundation of strength to do them safely.
As a tennis player, functionality and mobility are the cornerstones of what we do, so don't blow that by doing ab exercises that look cool, but that you shouldn't be doing until you are ready. Remember, it is in repetition of the fundamentals on a daily basis that true mastery is found.
So anyway, here is the third of the fundamental moves on the Swiss ball...it is called the Roll-Out.
Just like the the Stability Ball Crunch and SB Oblique Crunch (see last post), begin with 1 set of 12 repetitions and work your way up to 1 set of 20 repetitions. Work on these three exercises for 2-4 weeks and then you'll be ready to take it to the next level.
Train on Purpose and Play with Passion,