Hingis vs Capriati Aus Open 02. Pretty much down to the heat, both players having to be extensively iced down after the 2nd set. Stories of Hingis' body covered in goose bumps, crying on Capriati's shoulder cos she couldn't carry on. I'm surprised this match wasn't mentioned before actually.
We can all verify how heat can make us feel lethargic imagine playing the rallies these two were playing, definitley has mental affects aswell.