Tennis is a far more difficult sport, both technically and physically, than football. You have to put in vastly more hours of practice to achieve top level success. When you look at top class tennis players, the skill and cosistency is simply mind boggling. Far more people play football but you can get away with a lot less skill and dedication. The likes of Federer, Murray and Nadal would find it much easier to change careers and slot into top class football. A top footballer would have absolutely no chance of slotting into tennis.