It's not in Novak's hands at all.

He's defending 2600 points. W Shanghai, F Paris, F London

He added one more tournament, so he'll play Tokyo as well.

That means he has a chance to gain 1400 points. (W Tokyo, W Shanghai, W Paris, W London)

Let's say Rafa skips at least one of Shanghai/Paris, and the other one he reaches Finals, and let's say he gets lucky and Novak and Roger are in the same RR (let's say he goes 3-0 in RR), so he reaches London finals, but loses there.

That means he'll gain 600 from Shanghai/Paris + 1000 London = 1600 points.

With that, Novak would have 11265, and Rafa 10825. That is just 440 points.

That practically means that Novak has to play his perfect tennis and win Tokyo, Shanghai, Paris, London, while Rafa has to miss one tournament, and play at his usual level in other two.

Of course, if Rafa decides to play both Shanghai and Paris, he'll end the year as new World #1, barring some shocking losses in like first or second rounds.

If Rafa decides to skip both Shanghai and Paris, then Novak's chances grow significantly.

TL;DR It all depends on Rafa. If he plays tournaments, he'll finish as #1. But at the same time Novak has to play his perfect tennis to have a chance of staying there.