I think it comes down to motivation.
When Fed is really motivated, he will do everything in his power preparation wise to be ready for a match and a tournament. I think once Fed regained #1 you saw a little waning of his motivation.
It's hard to say how motivated Fed will be next year...I could see him trying to really pick his spots interms of peaking next year(even more then normal) and imo with his chances of winning grand slams dwindling I could see Fed going "all in" for another push at Wimbeldon.
To be honest what I could see next year is Fed winning 3 or 4 tournaments rather then 6. Still winning one slam if he moves heaven and earth to win Wimbly and ending up in the hi 50's for match wins. The days of winning 70 plus matches probably ended this season.
Still a stellar potential 2013 for the Fed though. Ending the year in 2nd or 3rd.
I am a HUGE Andy Roddick fan but I am aware that he has a Loopy, puff-ball FH. I Love Andy, but I'm realistic about how much he blows.