It is ok to wear contact lenses when you play tennis because you do not wear them for a long time, so they do no harm to your cornea. You could wear the contact lenses for 12 hours at most to make your eyes breathe. Make sure that your contact lenses are bought in the specialized glasses shops or hospital according to the situation of your eyes and radian of your eyes in order to avoid them dropping. Furthermore, as long as you wear them in a correct way and pay attention to the ocular sanitation, it does not matter. However, if you have astigmatism, thus wearing the contact lenses does not help, eventually you have to resort to the glasses, at least one of my friends who has astigmatism could not see the ball clearly even if wearing the contact lenses and still wear the glasses at last.