Year-end #1 scenarios now:
1. Nadal goes in with a 640-point lead. Each round-robin match is worth 200, semi-final is worth 400, final is worth 500. [N.B. That's a surprisingly small gap between semi and final. In Slams, semis are worth 480 points and finals are worth 800. In MS events, semis are worth 240 points and finals are worth 400. In 500-level events, semis are worth 120 points and finals are worth 200].
2. Unless Djokovic makes the final, he can't earn more than 600 points, and so unless Djokovic makes the final, Nadal is year-end #1.
3. If Nadal makes the final unbeaten, he is year-end #1.
4. If Nadal makes the final but loses a group match en route, Djokovic can be year-end #1 if he wins the title unbeaten.
5. If Nadal makes the final but loses two group matches en route, Djokovic can be year-end #1 if he wins the title despite losing a group match.
6. If Nadal loses in the semis without losing a group match, Djokovic can be year-end #1 if he wins the title, so long as he loses no more than one group match.
7. If Nadal loses in the semis having lost one group match or if he wins two group matches but doesn't make the semis, Djokovic can be year-end #1 if he wins the title (no matter whether he loses a group match or even two en route) but must win the title.
8. If Nadal loses in the semis having lost two group matches or if he wins one group match and doesn't make the semis, Djokovic can be year-end #1 if he makes the final, so long as he loses no more than one group match. In this scenarios, Djokovic is guaranteed to be #1 if he wins the title, even if he loses a group match or even two en route.
9. If Nadal wins no matches, Djokovic is #1 if he makes the final and wins two group matches en route.