Every year, the rankings break down differently. Every year, top players do either better or worse relative to each other, and we end up with different points necessary to earn a spot in the World Tour Finals at the O2 Arena in London.

Over the past few years (almost a decade, really), the number of points necessary to qualify was incredibly low. The reason was very simple. As the ATP “Big Four” of Roger Federer, Rafael Nadal, Novak Djokovic, and Andy Murray gobbled up obscene amounts of points, peaking at 84% of possible points earned in 2011, there just weren't that many other points to go around. In fact, in 2011, Mardy Fish qualified for London while earning fewer than 3,000  points on the year.
 
This year, though, that number is dropping. The Big Four have only earned 66% of the possible points this year. A large part of that may be due to Murray's struggles returning from injury and Nadal's absence at the US Open, but the fact remains that there are just more points to go around.
Over the past few years, a player needed around 3,300  points to earn his spot in the year-end top eight. Right now, Race No. 10 Grigor Dimitrov has 3,335  points on the year. Murray is not so far behind with 3,155  points already. And, while Murray marks the last serious contender as the gap between him and No. 12 Jo-Wilfried Tsonga is over 500 points, Tsonga and Ernests Gulbis are not quite out of the chase yet.
 
The last two months of the season offer a lot of points to be earned. Players can compete in up to two 500-level tournaments. Moreover, each of the weeks that hosts a 500 event features two of them, meaning that twice as many players can earn those points. Also, there are two Masters 1000 tournaments remaining, one in Shanghai at the beginning of October and one in Paris-Bercy at the end of the month to close out the season. In addition to that, there are 250-level tuornaments scattered in the weeks between them. Some top players will be competing in those, and in a race this close that could make a huge difference.
 
In theory, that means a player could earn 3,000  points between now and the end of the year, which basically puts anyone in the top 50 in striking range of the top eight. Realistically, no player towards the bottom of the top 50 will win that much, although it would be fascinating to see someone like Nick Kyrgios go on an absolute tear, but the point stands that the entire Race is wide open.
 
As of right now, four players have already clinched spots in the Finals in London. Djokovic, Nadal, Federer, and Stan Wawrinka have all punched their tickets, and it looks likely that Nadal will be healthy enough to compete by then. Marin Cilic sits at No. 5 in the Race with 4,000  points, making it very unlikely that he will fail to qualify, even if his results over these next few months are poor. While there are still a ton of points to go around, but it will be difficult for four different players to earn enough points to pass him. Basically, it would take the equivalent of the four players trailing directly behind Cilic winning one of each of the 500-level tournaments and adding an additional good result at a Masters (or a win at a 250) on top of that. It's not impossible, but it's very unlikely.

The Race really heats up when we look at those last three spots. Kei Nishikori's cushion over No. 9 Milos Raonic is only 250 points. Nishikori is playing in a 250 event in Kuala Lumpur this week, and a win there would give him some much-needed breathing room. Raonic and Dimitrov will be playing in 250s of their own later in the season, though, and would have a chance to gain those points back then.
 
In reality, though, this Race could come down to how these contenders will compete against each other. Most of the players in the Nos. 6-11 range could be drawn against each other in the quarterfinals of the Masters 1000 tournaments and either the quarters or semifinals of the 500s. These matches are worth a lot of points. For example, a Masters quarterfinal win is an additional 180 points. In a race this tight, those head-to-head matches could decide everything. Of course, the contenders have to get that far first, which is never a given in these tough tournaments.