Hmm actually are we over-egging this tiredeness thing a bit? Andy had a 3 (or was it 2?) week break after wimbledon. He then played what was basically a Masters tournament with no long matches. He had a few days rest before yesterdays match. Why should he be tired?
Because he's not had a few days rest, he's had about one (and that spent trying to adjust to the surface). There was two days between the final and the Toronto 2nd round, and he spent one of them flying.
He also has to deal with switching to a new surface, having had about a day to train. That's not easy.
Then there's also the emotional side, which you would imagine would also have an effect (from the elation of winning the Olympics to playing in front of a few hundred fans against a guy ranked #96 in the space of three days). Although with Lendl there and Andy's new-found mental strength hopefully that won't have too much effect.