Uhm, Nadal was tired not because of the previous match, it was an accumulated fatigue of sweeping most of clay titles, RG, Queens, Wimby, OG, Toronto, semis at Cinci. What was Murray doing during that time? Eating cooking and drinking warm milk at home. He was fresh and didn't play too many matches, especially on clay for obvious reasons.
Nadal did have weeks off after Wimbledon and a week before US Open. He went into that tournament flying after becoming #1 and winning gold.
For a 'tired' player he was unusually passive in the match and let Murray dictate throughout. The latter felt the full force of taking him down as he was pretty weary himself coming out against a fresh Fed, who did school him don't get me wrong.