The ATP World Tour Finals (also known as the Barclays ATP World Tour Finals for sponsorship reasons) is a professional men's tennis tournament played on indoor hard courts and is held annually in November at the O2 Arena in London, United Kingdom. The ATP World Tour Finals are the season-ending championships of the Association of Tennis Professionals (ATP) World Tour, featuring the top eight singles players and doubles teams of the ATP Rankings. The tournament was first held in 1970. The current champions (2015) are Novak Djokovic in singles and Jean-Julien Rojer and Horia Tecău in doubles.
Unlike all other singles events on the men's tour, the ATP World Tour Finals is not a straightforward knock-out tournament. Eight players are divided into two groups of four and play three round-robin matches each against the other players in their group. The two players with the best records in each group progress to the semifinals, with the winners meeting in the final to determine the champion. Though it is theoretically possible to advance to the semi-finals of the tournament with two round-robin losses, no player in the history of the singles tournament has won the title after losing more than one round-robin match.