Melbourne summer heat is very 'up and down'. You have your sunny scorchers, where it is just dry and HOT, and other times it can be the same temperature but very humid and 'muggy' where it is difficult to breathe! Other times the heat can be more pleasant especially when you get the light southerly breezes.
I dont know what US Open heat is like, but I imagine it is very tough anywhere to play in very hot conditions. The Rebound Ace courts here get very hot and sticky and some players have described it as being slowly cooked in an oven!