I used to do it all the time and than one day I got
heat exhaustion. I continued to play and sure enough a couple weeks later,
it happened again. I continued to play tennis later in the day or on less hot days, but ever since than I am sensitive to heat. Every summer I experience it, I get fatigue, headaches, nausea, and a general wiped out feeling on the hottest days. Even if I am not outside, if it gets very hot, it affects me.