Make sure you have plenty of water/ sports drink. If you feel thirsty, chances are, that you are already a bit dehydrated. Start drinking 2 hours before you even get on the courts. Dri-fit boxers/briefs by Under Armour are much more comfortable in hot weather compared to cotton underwear. Wear a Dri-fit Nike cap/ visor, along with some decent sports sunglasses. Keep a hand towel in your tennis bag for wiping down the sweat in between changeovers. Good conditioning is very important in combating heat. I'd recommend some running or swimming in the early morning or late evening (make sure you have good running shoes) and maybe one or two times in the gym for some weight lifting. A good diet is also important in managing heat and body weight (the less weight you carry, the better you will move). And don't forget the sunblock.
