I lived on the Mississippi Gulf Coast for 12 years or so. The typical temps during summer months, day after day, was 93F and the relative humidity was also 93%. At first I thought I was going to die. I did, mostly, get used to the heat eventually. I played tennis regularly in those days three or four times each week. I could play tennis all day but I did drink a great deal of water and never had a problem with cramping but I did sweat buckets. One of my frequent tennis partners was a born and bred Southern boy. When i was sweating copiously, he would get a light sheen on his brow.
On one occasion, work for several people at our composite shop took us for one week to Mississippi State University at Starkville which had a large autoclave to cook a project under pressure. The campus was several hours drive further North and not being subject to the temperature moderating influence of the Gulf of Mexico was hotter by 4 or 5 degrees F. Naturally I brought along tennis rackets to work on my twist serve. So after work one day I found the campus tennis courts and proceeded to pound serves. Within 10 minutes I felt ill and had to stop even though I did not feel it was that hot. I was surprised a few degrees more of heat would have such an immediate effect. I'm even more surprised that professional world class tour players can play five set matches lasting 4 hours or more, under the same conditions.