Factbox - The eight contenders at the ATP World Tour Finals

By Martyn Herman LONDON (Reuters) - The season-ending ATP World Tour Finals feature the top eight players in the rankings. 1 - Novak Djokovic (Serbia), aged 28, ninth appearance (champion four times) Ending the year playing tennis at a level no one can match. But for losing to Stan Wawrinka in the French Open final, his 2015 would have gone down as one of the greatest seasons in the sport's history. He won the other three grand slams and six Masters Series titles and has a vice-like hold on the world number one ranking. Almost unbeatable. 2 - Andy Murray (Britain), aged 28, eighth appearance Murray reached the Australian Open final and the semis at Roland Garros and Wimbledon, won Masters crowns in Montreal and Madrid and can end the year ranked two for the first time. Has never won the season-ender and may have his mind elsewhere with the Davis Cup final looming. 3 - Roger Federer (Switzerland), aged 34, 14th appearance (champion six times) Still the top ticket in town, the 17-times grand slam champion's popularity in London knows no bounds. No discernible drop in his level and his Wimbledon win over Murray was arguably the best tennis he has ever played. With his serve working like clockwork and his volleying skills more in evidence he will be the favourite to stop Djokovic. Won six titles this year. 4 - Stanislas Wawrinka (Switzerland), aged 30, third appearance The only player to beat Djokovic in grand slam play this year when his lethal single-handed backhand, one of the great strokes in tennis, stopped the Serb in the French Open final. Semi-finalist the last two years, Wawrinka will be expected to do at least as well this time. 5 - Rafael Nadal (Spain), aged 29, seventh appearance (qualified 11 times but missed four years because of injury) Finishing the year showing signs of a return to his best after a disappointing season in which he failed to win a grand slam for the first year since 2004. Slipped out of the top five for the first time since 2005 early in the summer and was beaten by Djokovic in the French Open quarter-finals. Good runs in Beijing, Shanghai and Basel made sure of his London spot, but no title since Hamburg in August. 6 - Tomas Berdych (Czech Republic), aged 30, sixth appearance The powerful Czech had another consistent year although again he spent most of it below the radar, picking up a couple of titles late in the season in Shenzhen and Stockholm. Beat a weakened Nadal at Australian Open. Reached the semi-finals in London in 2011. 7 - David Ferrer (Spain), aged 33, seventh appearance (played last year as alternate) Described his season as "unbelievable" after picking up his fifth title of the year by winning in Vienna. One of the most durable players on the Tour, he may not win many style points but will always give 100 percent and his game is under-estimated. 8 - Kei Nishikori (Japan), aged 25, second appearance The youngest of the eight qualifiers, Nishikori made the semi-finals on his debut a year ago but has struggled to live up to his 2014 breakthrough season when he made the U.S. Open final. Losing in first round at Flushing Meadows was a low point but he still picked up three titles and clearly has the weapons to challenge for majors in 2016. (Reporting by Martyn Herman; Editing by Ken Ferris and Toby Davis)