I had a match last Sunday at 11 am. Usually I try and refuse to play matches at this time, but I wanted to see how my fitness training would affect me.
I did much better than I normally would have, but it was still too hot to really have any fun at all. The guy I played is a grinder. Makes very little errors and forces you to earn every point. Not fun in 95 degrees with 80% humidity.
I tried using the frog towell thing. It works pretty well, but you have to keep in in ice water to keep it cool. IMO it's not any better than just dowsing your head with ice water if you have some, plus within 30 seconds you're just as hot as you were before. It still amazes me that people can play at this time of day and not be too bothered by the heat.