If Fed still wants to play tennis, he needs to work on some issues, yes, even at the age of 31. There's nothing much he can do with his physical game (except maybe work on his serve that failed him at WTF).
However, what Fed can do is work on his poor shot selection, analyze it and improve. I mean, just look at the first set TB against Nole. Fed just hit a glourious shot and manged to get back to 5-5. What does he do in the next point? Try to go for an impossible BH DTL on the first shot (where he knows the chances for making that one were very low) instead of trying to build a point.
Moving on to the 2nd set, Fed serves for the set at 5-4, 40-15. What does he do? Goes for the arrgoant FH inside out, it missed by very little, but nonetheless it missed. Fed goes on to lose 4 consecutive points (mostly on his own errors) to get broken. Then he gets broken again for the match after some horrible errors (missing by miles).
Now, if your not playing your best and can hit winners at a given moment, maybe go back to the basics instead of trying to force it? Why didn't he use the slice? The shot that always served him so well against Novak was barely used as an offensive tool, and instead he always tried to go toe to toe with the sometimes-great-but-mostly-inconsistent BH of his. The chip and charge that worked so well against Murray was gone, net game was barely there, and the serve failed him at the most crucial moments (usually its what saves him).
Fed could win this match, he had it, he played better than Novak for a big portion of the match, but him trying to go for highlight reels ultimately killed the match for him.