It does get cooler during the night after the sun goes down- but can still remain pretty darned hot in Austin and Houston. Dallas, Austin and Houston are all classified as humid subtropic, but Austin is three hours south of Dallas- so there is a difference imo in the cities in terms of weather. Austin and Houston tend to resemble each other more in terms of weather than either do Dallas. Texas actually has a large variety in terms of weather since the state is so large.I can recall heat indexes in southern Florida of over 120 degrees . . . the humidity here can be up to 95% at times. IIRC, night temps in Texas in the summer are lower than in southern Florida. We can stay up in the 90s at night. Florida's climate is considered "humid subtropic," except south of Lake Okeechobee, which is a true "tropical" climate. I don't know if Texas fits that description. My recollection from visiting my uncle in Dallas many, many years ago is that once the sun set, the temperature really dropped.
All I know- is that it can get pretty darned hot here (in 2000 there were more than 40 days above 100 degrees) and that folks sometimes do make an effort not to play tennis in really hot conditions if they can avoid it. Same thing with other activities. You will find much fewere people on the hike and bike trails during the warmest part of the day than you will when it is a little better. As you and others have said- people do play in extreme conditions- but sometimes adjustments are made for the season.