He was tired though seriously, you noobs can't admit that. Murray lucked his way to the final in US Open 2008.....but I think he earned his runs to the finals in AO 10 and 11.
In the penultimate point when he got battered all over the court he did look totally out of breath, but besides that, he was fine. Get over it.
A more key factor was the first two sets being on the much faster Armstrong court which favoured Andy.