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 Tecau in doubles.
Unlike all additional singles events on the men's tour, the ATP World Tour Finals isn't a straightforward knock-out tournament. Eight players are divided into two groups of four and play three round-robin matches each against the additional 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.
The current round robin format of two groups of four players progressing to a semifinal and final, has been in place for all editions of the tournament except the following years:
1970, 1971 - Round robin with no semifinals or finals, winner decided on best performed player
1982, 1983, 1984 - 12 player knock-out tournament with no round robin. The top four seeds in the event received a bye in the first round.
1985 - 16 player knock-out tournament with no round robin
In the current tournament, winners are awarded up to 1500 rankings points; with each round-robin loss, 200 points are deducted from that amount.
The event is the fourth evolution of a championship which began in 1970. It was originally known as the Masters Grand Prix and was part of the Grand Prix Tennis Circuit. It was organised by the International Lawn Tennis Federation (ILTF) ITF. It ran alongside the competing WCT Finals the additional season ending championships for the rival World Championship Tennis Tour. The Masters was a year-end showpiece event between the best players on the men's tour, but didn't count for any world ranking points.
In 1990, the Association of Tennis Professionals (ATP) took over the running of the men's tour and replaced the Masters with the ATP Tour World Championship . World ranking points were now at stake, with an undefeated champion earning the same number of points they would for winning one of the four Grand Slam events. The ITF, who continued to run the Grand Slam tournaments, created a rival year-end event known as the Grand Slam Cup, which was contested by the 16 players with the best records in Grand Slam competitions that year.
In December 1999, the ATP and ITF agreed to discontinue the two separate events and create a new jointly-owned event called the Tennis Masters Cup . As with the Masters Grand Prix and the ATP Tour World Championships, the Tennis Masters Cup was contested by eight players. Notwithstanding player who's ranked number eight in the ATP Champion's Race world rankings doesn't have a guaranteed spot. If a player who wins one of the year's Grand Slam events finishes the year ranked outside the top eight but still within the top 20, he's included in the Tennis Masters Cup instead of the eighth-ranked player. If two players outside the top eight win Grand Slam events, the higher placed player in the world rankings takes the final spot in the Tennis Masters Cup.
In 2009 the Masters was renamed to the ATP World Tour Finals and got scheduled to be held at The O 2 in London from 2009 to 2013.  In 2012 the organisers extended the contract by two years up to 2015.   For a large number of years, the doubles event was held as a separate tournament the week after the singles competition, but more recently they have been held together in the same week and venue. Like the singles competition, the doubles involves the eight most successful teams on the tour each year, and starts with a group phase with each team playing three round-robin matches.
For most of its history, the event has been considered as the most important indoor tennis tournament on the world tour (there were a few exceptions, when the event was organised outdoors: 1974 Melbourne & 2003-2004 Houston), allowing for controlled conditions of play, regarding both surface type and illumination system.
Roger Federer holds the record for the most singles titles, with six. Federer additionally holds the record reaching the final title match the most times in this tournament, with ten.
Points, prize money and trophies
The ATP World Tour Finals currently (2015) rewards the following points and prize money: 
|Round Robin win per match||$167,000||$32,000||200|
|Participation fee||$167,000 2||$82,000 3||–|
- 1 Prize money for doubles is per team.
- 2 Pro-rated on a per-match basis (2015 split was $95,000 = 1 match, $125,000 = 2 matches, $167,000 = 3 matches)
- 3 Pro-rated on a per-match basis (2015 split was $32,000 = 1 match, $60,000 = 2 matches, $82,000 = 3 matches)
The tournament has traditionally been sponsored by the title sponsor of the tour; however, from 1990–2008 the competition was non-sponsored, even though the singles portion of the event as part of the ATP tour was sponsored by IBM. In 2009, the tournament gained Barclays PLC as title sponsor.  Barclays confirmed in 2015 that they wouldn't renew their sponsorship deal once it expires in 2016. 
|Tokyo||1970||Carpet||Tokyo Metropolitan Gymnasium||6,500|
|Paris||1971||Stade Pierre de Coubertin||5,000|
|New York City||1977–1989||Madison Square Garden||18,000|
|Hanover||1996–1999|| Carpet (1996) |
Indoor Hard (1997–99)
|Lisbon||2000||Indoor Hard||Pavilhão Atlântico||12,000|
|Houston||2003–2004||Outdoor Hard||Westside Tennis Club||5,240|
|Shanghai||2005–2008|| Carpet (2005) |
Indoor Hard (2006–08)
|Qizhong City Arena||15,000|
|London||2009–2018||Indoor Hard||O2 Arena ||17,500|
Singles finals matrix
|Titles||Player||Years Won||Years Runner-up|
|6||Roger Federer||2003, 2004, 2006, 2007, 2010, 2011||2005, 2012, 2014, 2015|
|5||Ivan Lendl||1981, 1982, 1985, 1986, 1987||1980, 1983, 1984, 1988|
|Pete Sampras||1991, 1994, 1996, 1997, 1999||1993|
|Novak Djokovic||2008, 2012, 2013, 2014, 2015|
|4||Ilie Năstase||1971, 1972, 1973, 1975||1974|
|3||Boris Becker||1988, 1992, 1995||1985, 1986, 1989, 1994, 1996|
|John McEnroe||1978, 1983, 1984||1982|
|2||Björn Borg||1979, 1980||1975, 1977|
|Lleyton Hewitt||2001, 2002||2004|
|1||Andre Agassi||1990||1999, 2000, 2003|
|Stan Smith||1970||1971, 1972|
|0||Jim Courier||1991, 1992|
|Vitas Gerulaitis||1979, 1981|
|Rafael Nadal||2010, 2013|
|Juan Martín del Potro||2009|
|Juan Carlos Ferrero||2002|
Doubles finals matrix
|Titles||Player||Years Won||Years Runners-up|
|7||Peter Fleming||1978, 1979, 1980, 1981, 1982, 1983, 1984|
|John McEnroe||1978, 1979, 1980, 1981, 1982, 1983, 1984|
|4||Bob Bryan||2003, 2004, 2009, 2014||2008, 2013|
|Mike Bryan||2003, 2004, 2009, 2014||2008, 2013|
|Daniel Nestor||2007, 2008, 2010, 2011||1998, 2006|
|3||Anders Järryd||1985, 1986, 1991||1989, 1992|
|Rick Leach||1988, 1997, 2001|
|2||Todd Woodbridge||1992, 1996||1993, 1994|
|Mark Woodforde||1992, 1996||1993, 1994|
|Max Mirnyi||2006, 2011||2009, 2010|
|Jacco Eltingh||1993, 1998||1995|
|Paul Haarhuis||1993, 1998||1995|
|Nenad Zimonjić||2008, 2010||2005|
|Stefan Edberg||1985, 1986|
|Jonas Björkman||1994, 2006|
|1||Sherwood Stewart||1976||1982, 1984|
|John Fitzgerald||1991||1989, 1992|
|Mark Knowles||2007||1998, 2006|
Most titles: 
Most consecutive titles:
Most consecutive finals:
Players who won the tournament undefeated:
Note: in 1982–85 there was no round robin, just sudden death
- Roger Federer, 14 (2002–2015)
- Andre Agassi, 13 (1988–1991, 1994, 1996, 1998–2003, 2005)
- Ivan Lendl, 12 (1980–1991)
- Boris Becker, 11 (1985–1992, 1994–1996)
Jimmy Connors, 11 (1972–1973, 1977–1984, 1987)
Pete Sampras, 11 (1990–2000)
Most Consecutive Appearances:
- Roger Federer, 14 (2002–2015)
- Ivan Lendl, 12 (1980–1991)
- Pete Sampras, 11 (1990–2000)
- Novak Djokovic, 9 (2007–2015)