OK, so Andy has a path to #1. It is narrow, but possible. He's 915 points behind Novak. What might that path look like?
First of all, neither seems to be playing an ATP 250 next week. Presumably both will play in one of the two ATP 500s in the following week, the Paris Masters, and the World Tour Finals - a total of 3000 possible title points (or 2700 if not undefeated at the WTF). Sounds like a lot, but not a lot to make up 915 points.
Essentially Andy has to do equal or better than Novak at every tournament, and then win the WTF. Here are the point ranges at each tournament:
ATP 500: 500, 300, 180, 90, 45, 20, 5
Masters: 1000, 600, 360, 180, 90, 45, 25/10
WTF: 200 each RR win, +400 for final, +500 for title - so: 1500/1200 for win, 1000/800 for final, 600/400 for semifinalist, 200/0 for RR.
To make the year-end #1 a realistic proposition, Andy needs to win the WTF. Let's say he wins but with one defeat, and Novak is the finalist. That means Andy gets 1200 points to Novak's 800 or 1000, so Andy will get +200 to +400. That means he needs to win one or both of the ATP 500 and Paris, or at least do significantly better than Novak. If he wins WTF undefeated, and Novak loses once in the RR but still makes the final, he is +700 over Novak, so needs a slimmer margin in the earlier tournaments.
Either way, each tournament counts - and there's a nice leadup, with an ATP 500, a Masters, than the WTF. If Novak wins an ATP 500, it isn't utter disaster for Andy (unless he goes out really early) but is a big setback. If Novak wins Paris, it is over. So again, the key for Andy is to do equal or better at both, and better at at least one, and then win the WTF.