The Association of Tennis Professionals calculates tennis world rankings by totaling the points that each player earned in various competitions, including Grand Slams, ATP Challenger Tour and Futures tournaments. As of 2015, ATP takes into consideration the immediate past 52 weeks relative to the date publishing the world rankings.
At the end of a tennis season, ATP calculates points from four Grand Slams, eight mandatory ATP World Tour Masters 1000 tournaments and six best results that each player earned from ATP World Tour 500, ATP World Tour 250, ATP Challenger Tour and Futures tournaments. In case a player doesn’t participate in Grand Slams and ATP World Tour Masters 1000 tournaments, ATP increases the number of results from other tournaments that qualify for the world rankings by one. During ranking periods without major tournaments, ATP adjusts the number of best results from other tournaments accordingly. If a player gets accepted in the main draw of a Grand Slam or an ATP World Tour Masters 1000 tournament, ATP takes into consideration the achieved result whether that player participates in the tournament or not, even if he has to skip the tournament due to an injury.
ATP uses the tennis world rankings to determine seeding and entry for most singles and doubles ATP tournaments. The only exception is the Barclays ATP World Tour Finals tournament, for which ATP uses modified rankings.