From the USTA:
"Q. How much fluid should I drink before training and when? Also how much fluid should I drink while I am training and what should I drink water, powerade etc? And do you know two ways to determine my own hydration level? Will water alone be enough for players to compete at their best and allow full recovery?
A. The USTA Sport Science Committee has put together fluid replacement guidelines and this information is available on the High Performance website.
In general, you want to:
Top off your fluid stores by drinking 12-16 oz of fluid 1 hour prior to competition.
Drink 4-8 swallows of fluid after your warm-up and on EVERY changeover during play.
Drink 20-24 oz of fluid for every pound of body weight lost during play to replenish the body's fluid stores.
Whether to use water or a sports drink is something you have to determine on your own. The three things you should be replacing during or after play are fluids, energy/ carbohydrates, and electrolytes.
Many sports drinks contain all of these, but you can also obtain these through a combination of drinking water and eating appropriate snacks. Some people have difficulties with sports drinks, so the later suggestion may be for you if drinks like Gatorade or Powerade are hard on your stomach.
Two simple ways you can determine your hydration status are:
1. Monitor the color of your urine. In a well-hydrated person, urine will be very light to clear - the color of straw. Darker, more concentrated urine is an indicator that you are dehydrated.
2. Weigh yourself before and after play. Any weight loss you experience during play is due to fluid loss and that weight should be replaced before the next practice or match. Make sure when weighing yourself, however, that you wear similar DRY clothes, and not the sweaty clothes you came off the court in."