A. Roddick has virtually no points to defend except his QF. He will gain points this season. I think he also has maybe a QF or SF in Paris, I don't remember.
B. Del Potro is playing even better than last season. If he draws Djokovic in his QF, he's winning. He played fairly subpar at the end of the season and the YEC, so he can still manage to pick up points there. Oh, I'm also not sure how well he did at the Masters (Canadian/Cincinnati) either, but it wasn't too good from what I remember.
C. Djokovic cannot defend Olympic points, has to defend Cincinnati and Montreal points, including his USO SF. He also has to defend Madrid (Or Shanghai) and Paris points, on top of his TMC points. In fact, if you total up the amount of points he has to defend, he has something like over 3000 points to defend. With his current form, he can easily lose half of those if not mroe.
In fact, what seperates Del Potro, Roddick, and Djokovic is really the TMC victory for Djokovic. Subtract that and there's really not much of a difference. I think what seperates Djokovic and Del Potro if you remove the TMC (which Djokovic will very likely not defend) is about 900 points total. If Del Potro does well in the HC season and Djokovic slumps, it's not out of the possibility he could drop.