Andy has always had problems with heat, remember the Lopez match at last years US Open?
It's a different kind of heat really. In the west like California it's dry and searing. However in the South the heat is humid and really bad for your hair.
So I'm guessing he has a hard time in dry heat which can be energy sapping.
Anyway Tomic was his usual self and Andy stayed calm. Nice win, and now it's time for another cute youngster.