Virtually mathematically impossible for him to finish as #1.
Well, he could finish as low as #5, as you suggest. However, I don't rate Djokovic as highly on the quicker surfaces (compared to medium-paced hard courts and clay)...I think he is a little more prone to earlier exits (than SF), which would hurt his chances a little.
However, the threat from Del Potro is very real...he can overtake Muzza just by playing the Tokyo final next weekend. And playing catch-up with Delpo isn't easy, considering how consistently solid he has been in posting good results in the biggest draws this year.
I think that, on paper, Muzza is a little bit stronger than these two on the quicker surfaces. Usually, you'd be looking out for him to go that extra distance and play the key title match(es).
However, I do have two key concerns...and I fear that we could see both become reality over these last weeks of the season:
1. Fitness - if Muzza extends his lay-off beyond Tokyo, he is likely to seriously lose out. But, obviously, he is wise not to return until his wrist is in good enough condition...
2. Game style - if Muzza continues to play the predominantly 'pushy' tennis we have seen of him over the last few months, he'll be checking out of hotels earlier than many would be expecting. Anyone thoughtful enough to reserve a couple of taxis for Castle and Reed...?
Looks like the World Tour Finals could witness the climax of this rankings battle.