If you feel thirsty, this is the first warning of dehydration; your body is using up not only water, but the components which make up the electrolytes your body uses to function. If you lose too much fluid, you can go from hot to heat stroke very quickly while exercising, especially with high heat and humidity.
What is the recommendation for tennis players? Start early! By drinking a sports drink, taking electrolyte tablets, etc. a half hour before beginning to work out, your body will be prepared to withstand the summer heat. In todays market, you can find many options for electrolyte replenishment; Smart Water, Gatorade G2, Nuun Electrolyte tablets, etc. which are specifically designed for the sport you play.
Once you begin to work out, take periodic breaks to replenish your electrolytes and remain in a shaded area. Water gets in and out of your system very quickly. Drinking a Gatorade once you’re already working out will quench your thirst, but the benefits of the drink won’t begin to work until 15 to 30 minutes depending on how full your stomach is. For players with cramping issues, a sport drink with free radicals and potassium can slow the build up of lactic acid in your muscles which causes these issues.