He has to be the favorite considering he has won it three of the last four times. For rock people, that means he already owns three AO titles AND the other two Slams (Wimbledon and the US Open) have been won just one time a piece. Nadal, last year's finalist with Djokovic, who extended the contest to a five setter that set a record for 'time on court' so emphatically that the two contestants couldn't even stand for the award ceremony, is not playing this time. Federer is now 31 and is expected to drop from at least the Slam contention ONLY if Djokovic fails to do what is expected of a top player in his prime. Sure, Murray is coming off an inspiring US Open win but not many believe he is STILL versatile enough to challenge an in-full-flight and far more talented Djokovic at ANY Slam.
Yet, despite above, the margin is not enough to overwhelmingly declare Djokvoic the outright winner, largely because of Djokovic's own inability to sustain top form through an entire season. I mean, it's not even close to, say, Federer contesting @ Wimbledon in his prime or Nadal @ Roland Garros. There are ample chinks for BOTH Federer and Murray to exploit.
Besides, Murray may prove to be the loose canon with potential to decide the winner. How? By electing to fall on one side of the draw. Beating Murray AND the finalist (Djokovic or Federer) may prove to be a tall order on it's own. But when you add the massive discrepancy in resistance the other finalist will face in his semis, you can hardly resist crowning the 'other' guy the winner - already - don't you think? Even for Murray to fight both cats back to back is a tall order. He won the 2012 US Open facing Berdych in the semis, not Federer. And yet it took him a close and gruelling five setter to close the deal.
Sure, Djokovic did'n't face Federer in the semis either (Ferrer) but Djokovic is the incumbent and therefore the onus to produce under stiffer conditions, specially when you have just ONE fall-back precedent, is on you. So in layman terms, the Murray semifinal bracket could guarantee a runner-up with winner required to just nudge the tired opponent in the final.
As far as Federer is concerned, he may not be wise to look past the quarterfinal and then take one point at a time moving forward. If he faces Murray in the semifinal, the 'time on court' stat registered by then WILL prove decisive given No. 31, heat and the far easier road Djokovic would have traveled highlighted by his rampage at Abu Dhabi few weeks ago, where he bageled Ferrer in the semis.
If 110% healthy, Del Potro may be the ONLY other human who can alter the above dynamic with any serious implications.