Yeah, Dallas is right - if Roger wins the US Open, he would be the favourite for the year end #1. Last year, he lost in the semis, so he earned only 720 points. So if he could win and earn the full 2000 this year, that would be a massive gain for him of 1280 points. Assuming Djokovic reaches the final, he would earn 1200 points, thereby losing 800 points in comparison to last year. Hence, a total points swing of 1280 + 800 = 2080. He already has a lead of almost 900 points, so winning the US Open would virtually ensure him the year end #1.
If Djokovic beats him in the final, Federer would still gain 480 points from last year, giving him an overall lead of 1375 points. Djokovic could grab the year end #1 if Federer underperforms at Shanghai, Paris or London.
If Murray beats Federer in the semis and Djokovic wins the final, the situation would remain the same as of now. Federer would still have a lead of 895 points, and it would be easier for Djokovic to overtake him at Shanghai or Paris.
If Murray beats Federer in the semi AND Djokovic in the final, he will become #3 while the gap between Federer and Djokovic would widen to 1695 points.