You have to be properly hydrated to withstand high intensity training, such as tennis. The value of water, your body’s most important nutrient, cannot be underestimated. Even a mild dehydration, as little as 2% loss of fluids, which is considered “normal” for humans, can have a detrimental outcome on your tennis performance. You don’t even feel thirsty until maybe at 3% of dehydration. At that time your performance is already impaired.