The first thing is that you can not trust your body to tell you when you have lost a dangerous amount of water. The thirst mechanism does an extremely poor job at gauging how much water your body needs. That is not too much. If you have ever seen anyone pass out for no apparent reason at the pool or maybe while watching a football game out in the sun, dehydration may have been the cause. If a tennis player plays more than one match in a day and does not replenish fluids properly, the effects could be disastrous.
Tennis players change their shirts often for a very important reason.
Most of you should already know that when you exercise, your body cools itself down by sweating. Sweat is a mixture of water, sodium, and potassium. That is why sweat may taste salty to you. Anyway, by secreting water onto the surface of the skin, the heat energy in your body can get transferred into the water and evaporate away. This is how sweat acts as a mechanism to keep your body's temperature cool during exercise.