Here is my logic behind the selection, both positive and negative.
Negative: I am ruling out Fed to win at all, based on his form, his age,
his back pain, racquet changing confusion etc. I think Murray
will be too complacent with two GS (including Wimbledon)
under his belt and so will falter. I think Djokovic is lacking
confidence due to the two recent GS losses (in both of
which he had reasonable chances apriori) and it is tough
to win it out without confidence. Nadal, may have peaked
too early. In particular, he was serving unbelievably in
Cincy. I expect a considerable deterioration in his serve
at USO (just because nothing stays the same), which will
lead to his loss. ATP is still not wide open like WTA for
an equivalent of Marion Bartoli to win. Hence, I pick the
player with best credentials outside the big four,
the tall humble man (who has Pope's blessings to boot
Positive: JMDP got his first GS in 2009 USO. In 2010, he was out
of action completely (played barely 3 tournaments) due
to wrist surgery. In 2011, he worked his way up in
ranking from infinity to top 100. In 2012, once inside
top 100, he worked his ranking way up to be a member
of top 10. So, finally he is now in a stage where he could
sit on the table with big four and fight with them. He has
won matches against every one of big four. Most recently
he did a fine job at Wimbledon against Djokovic. I am
assuming that he is fully healthy now.
General: Even though there is huge division between the rich and
poor in ATP, there is a parity reached among the rich folks,
as evidenced by four different people winning the four slams
in 2012. I expect this trend to continue for the next one to
four years, without a single player clearly dominating over
everybody else. As Fed is fading (I am sure he will come
back next year), I would give the last prize of this year
to Del Potro, the Tiger.