If a tennis ball hits you very hard in the abdomen, could that cause an abdominal organ to rupture?

Extremely unlikely: There would have to be an intrinsic weakness of an abdominal organ for it to rupture from even a hard hit tennis ball. If you have no bleeding disorders or abdominal organ malignancies, then the hit is just going to cause local pain, a little bruising but no internal damage.

Highly unlikely: While a professional player may be able to launch a ball at 130mph, the ball is soft enough to only cause minor bruising.