I believe even with all possible permutations it's in Novak's hands.Federer can only defend points where Novak can gain.
Federer is one of the greatest indoor players ever so if he's up for it he can hang onto the lead he has.
Novak still has quetions over him about performing for an entre season.However if he had won Wimbledon or the USopen he probably would have tailed off at this stage and played second fiddle.
After a season that promised much after Australia he has gone on to miss all of his mid season goals.Now he's motivated and probaly not as physically spent as last season.
Federer has complained of feeling wounded very surprising considering what he has manged to win the last 12 months.I think he was too hard on himself for losing at Flushing Meadows.
It comes down to the events they both pick and the mindset of the 2.
Who wants it most.