I think after winning wimby and taking the last important record, and losing olympics, Fed has lost motivation considerably.
The last I checked, the 2012 Western & Southern Open Cincinnati Masters 1000 was after Wimbledon and the Olympics, and arguably, Roger Federer played the best he has played this year for an entire tournament there. He faced only 3 break points the entire tournament and saved them. He won all 3 tiebreakers he played against Fish, Wawrinka, and Djokovic. Fish and Wawrinka may have played their best matches of the year in a losing effort to Federer, and Djokovic got bageled (though Nole's poor play contributed to that).
I would say that Federer probably peaked that tournament, and would agree that he has not been generally as good since then, probably due to a combination of mental and physical fatigue and attention for his time since regaining #1. Since the US Open this year, he has not been able to fence in his drops in focus and turn matches around as he had for almost 10 months or so since last year's Basel. He has also lost 2 key tiebreakers to Del Potro rather badly. One to lose the final in Basel, and one at London's O2 in the 3rd RR round in the first set. But, he did reach the final in Basel, and had flashes of brilliance here and there in his matches at O2 so it's not like he has totally fallen apart. His only losses since and including the US Open, have come to Berdych, Murray, and Del Potro, all tough players for him at times in the past.
Andy Murray's and Novak Djokovic's standard also hasn't been very good since Shanghai. They both performed poorly in Paris for whatever reason. Then at O2, Djokovic played poorly in the first round, but was very fortunate to be playing an even worse Tsonga. Since that match, he has slightly improved, the match Murray and Djokovic played was full of unforced errors (84 - D40, M44) and was lost more than won, and his best match thus far was against Berdych, an almost perfect match-up (HTH 11-1). Del Potro and Ferrer have probably been playing the best level indoors of any of the participants since Shanghai, Ferrer only losing to his worst match-up, Federer, and Del Potro losing to Ferrer.
We will see who can manage to pull their game together for the semifinals to go on to the final. Probably Del Potro and Djokovic appear to have the most confidence of the remaining participants. Djokovic cannot afford to play a poor first set against Del Potro, and should count on his great defense to frustrate Juan Martin. I wouldn't be surprised at any outcome with regard to Murray vs. Federer. They have both played up and down during their matches here. It just depends on who is more up than down for the longest tonight.