Novak Djokovic, the current World No. 2, stated shortly after losing the 2012 US Open final that regaining the World No. 1 ranking was "one of the objectives" he had for the remainder of the 2012 season. As of the October 1st, 2012 rankings Djokovic was 1335 ranking points behind Roger Federer, the current World No. 1, and to some that lead might look sizeable. But since Federer played so well late in 2011 and since Djokovic played relatively poorly, the Serb has a strong chance to regain the top ranking on tour before the start of 2013.
As most tennis fans know the ATP employs a 52 week rollover system when determining the rankings. There are times when things can be moderately complicated but basically a player stands to lose or gain points in any particular week depending on how he does in the week at hand in relation to how he did at the same time last year. When you look at the ranking points that Federer has to defend for the balance of 2012 and compare them to the ranking points that Djokovic has to defend, it clearly shows that the Serbinator has a realistic chance of overcoming the Swiss Maestro before the end of the current season.
Compare the following sets of statistics for the two players (all ranking points taken from ATPWorldTour.com's player profiles).
Federer late 2011 results:
Shanghai Masters 2011: Did not play - 0 ranking points
ATP Basel 2011: Winner - 500 ranking points
Paris Masters 2011: Winner - 1000 ranking points
ATP World Tour Finals 2011 (London): Winner - 1500 ranking points
Total ranking points to defend for the balance of 2012: 3000
Djokovic late 2011 results:
Shanghai Masters: Did not play - 0 ranking points
ATP Basel 2011: Semifinalist - 180 ranking points
Paris Masters 2011: Quarterfinalist - 180 ranking points
ATP World Tour Finals 2011: Round robin - 200 ranking points
Total ranking points to defend for the balance of 2012: 560
In order for Djokovic to overtake Federer the Serb would have to gain 1336 ranking points on the Swiss star and not just in the tournaments listed above. Currently Djokovic is active in the ATP Beijing draw, an event that isn't exactly loaded with top talent. If Djokovic can take that title down he would take down 500 ranking points for a pure net gain of 500 points. While he's only in the second round (at time of writing) David Ferrer, the number two seed in the tournament, has already been eliminated from the draw and that makes the Serb a bigger favorite than he was pre-tournament.
Federer and Djokovic both have all to gain at the 2012 Shanghai Masters since neither player played in that 1000 series event last season. But following Shanghai all of the pressure will be on Fed: Djokovic can gain tons of points in Basel, Paris, and London while Federer has so many to lose. That situation makes the Swiss Maestro's hold on the World No. 1 ranking quite fragile heading into the next month and a half.
If Andy Murray, the current World No. 3, keeps playing well then he can also throw himself into the mix for the World No. 1 ranking. However seizing the top spot on tour, if it is to happen, still looks at least a little ways off for the Scot. Murray won both Tokyo and Shanghai last season and that means his potential for growth isn't as large as Djokovic's is for the balance of 2012. The reigning US Open champion can make himself more of a contender with strong results throughout the balance of this season and then with a better Australian Open than what he turned in at Melbourne Park in 2012 (semifinalist).
Nadal, the current World No.4, is a very peripheral threat given his current playing status. According to a September 25th article at CBC.ca the 2013 Australian Open is "not even a sure thing" for Rafa due to his knee injury. However even if he got back to playing competitive tennis by Melbourne Park in January, with only 45 ranking points earned at both Wimbledon 2012 and the 2012 US Open combined, even the Nadal true-believers out there have to admit that the World No. 1 ranking looks out of reach until at least the summer of 2013.
Prediction for the season end rankings: 1. Djokovic, 2. Federer, 3. Murray