It is OK for you to wear the contact lenses when you play the tennis if you wear the glasses with no prescription to protect the eyes from getting hurts. Because the balls may knock on the eye surrounding parts, you will have to protect the eyes parts. However, you should better use the headband to tie on the head which will stop the sweat from coming into the eyes which will make the eyes feel sticky.
It seems that you are quite interested in playing tennis. So, actually, you are allowed to wear contact lenses as a means of vision aid, and there is no harm generally speaking. However, playing tennis would generate a lot of sweat, which is not good for the contacts that you wear. You should be careful not to let too much sweat get into your eyes, which could give rise to some uncomfortable feelings.
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.