I used to play juniors and while I wore shorts when training, skirts were much more comfortable, especially for running. I never played a tournament in shorts, they bothered me. Plus, the tights you wear under the skirt are great for putting the extra ball when serving.

Just wanted to say that Serena has a unique outfit. She never wears the same clothes as the rest of the girls. Maria Sharapova (pre mildronate scandal) used to have her own line as well.