The Broz strongly alluded to it, I must amplify: drink lots the day before!
Lead up to strenuous days in the heat by prepping your body for a few days ahead. I make it a lifestyle choice to just drink loads of water all the time (except on long drives). I remember one tournament in 101* (fahrenheit) heat where by the end of the day, it looked like everyone was moving in slow motion, and I was able to play quite well, if not comfortably.
Also, love that Under Armour. When you take off the jersey between games, it is cooler than naked! Looks slightly festive, but who cares.