Most tennis players know that they play better when they are relaxed. In fact, step #3 of the 4-D System (see my book: Deconstructing Tennis) makes touching base with a relaxed body/mind a part of the preparation between each and every point.
Playing competitive tennis can be stressful. And you all know that stress, beyond a certain level, makes any player perform well below her level. (I’m still waiting to hear from anyone on any subject whatsoever where excessive stress is beneficial!).