Depends on the player.
For Nadal or Djo yes but not Fed...he plays so effortlessly and dictates the game which keeps him fresh especially on grass.
if so why does he have such a poor record in 5-setters ?
why did he lose to Tsonga and Djokovic last year after leading 2 sets to love ?
funny that until recently people said "Fed is finished for 5-setters" and now some go to the absolute opposite.
OF course Fed gets tired like all the players
As for Murray, with Robson they will probably be defeated by Hewitt-Stosur anyway, and even if he plays two doubles, that's not such an important effort except if he plays 18-16 like Llodra-Tsonga.