Djokovic will most likely lose #1 around the middle of next year after Andy gathers champion points from the Australian Open, some masters tournaments and probably Wimbledon. Until then, his points total is too big for anyone to overtake him, unfortunately.
Indian Wells, Rome and Madrid are the key to this equation. That's where Murray needs to shine to get there. First though, he needs to completely take advantage of the fact that Federer has many points to defend this first third of the year. If he's second by the time we reach the clay season MM, there is chance. All in all though, it's not that easy at all.