The top tennis players in the world have descended on London as they prepare to compete at the ATP World Tour Finals at London's O2 Arena.

Rafael Nadal is a notable absentee after undergoing appendix surgery, but the tournament is littered with star names as defending champion Novak Djokovic, as well as six-time winner Roger Federer, look to add to their tally at the end-of-season championships.

Andy Murray will lead the British challenge, while this year's tournament will also see the debuts of Kei Nishikori, Milos Raonic and Marin Cilic after breakthrough years for the trio.

Below, Sports Mole takes a look at the eight contenders ahead of the start of the event on Sunday afternoon.

Group A

1. Novak Djokovic

Seed: 1

Best results in 2014: Wimbledon champion, Roland Garros runner-up, Winner at Indian Wells, Miami, Rome and Paris

Review: By his own high standards, Novak Djokovic began 2014 slowly after he failed to reach the final at the Australian Open and in Dubai, but the Serbian responded with back-to-back triumphs in Indian Wells and Miami. That was followed by victory in Rome before he came up short once again in his attempt to win at Roland Garros after losing to Nadal in four sets.

However, three years on from his maiden success at Wimbledon, Djokovic repeated the feat at SW19 by beating Federer in an epic final but a disappointing American swing ended with a shock semi-final defeat to Nishikori at the US Open. The world number one has regained his momentum, though, and wins in Beijing and Paris have set him up nicely for his return to London.

ATP Finals best performance: Winner (2008, 2012, 2013)

Opening match: Cilic on Monday evening

2. Stanislas Wawrinka

Seed: 3

Best five results in 2014: Australian Open champion, Winner in Monte Carlo and Chennai, Quarter-finalist at Wimbledon and US Open

Review: 2014 couldn't have started any better for Stanislas Wawrinka. He followed a win in Chennai by claiming his first Grand Slam title at the Australian Open against an out-of-sorts Nadal. The Swiss struggled on the back of that success but he rediscovered his form with success at the Monte Carlo Masters.

Quarter-finals at Wimbledon and the US Open have followed but that is as good as it has got for Wawrinka, who has not reached the semi-finals of any tournament since Queen's in June, and he will be hoping that a return to the English capital can inspire him to a positive end to the year.

ATP Finals best performance: Semi-finals (2013)

Opening match: Berdych on Monday afternoon

3. Tomas Berdych

Seed: 6

Best five results in 2014: Winner in Rotterdam, Semi-finalist at Australian Open, Miami, Paris, Quarter-finalist at US Open

Review: Tomas Berdych registered wins in Rotterdam and Stockholm during 2014 but it has been his overall consistency that has seen him remain in the top eight of the rankings.

As well as reaching the semi-finals at the Australian Open and quarters at Flushing Meadows and Roland Garros, the big-hitting Czech has also reached the final in ATP 500 events in Dubai and Beijing, and that ensured that he had more than enough points for another appearance in London.

ATP Finals best performance: Semi-finals (2011)

Opening match: Wawrinka on Monday afternoon

4.Marin Cilic

Seed: 8

Best five results in 2014: US Open champion, Winner in Zagreb, Delray Beach and Moscow, Quarter-finalist at Wimbledon

Review: Marin Cilic will himself acknowledge that he is only at the ATP World Tour Finals because of his ruthless displays at the US Open, but wins in Zagreb, Delray Beach and Moscow have proven that the 26-year-old is more than a one-hit wonder.

A quarter-final at Wimbledon also helped his cause, but Cilic will be keen to produce his best tennis on this platform to establish himself as a major player next year.

ATP Finals best performance: Debut

Opening match: Djokovic on Monday evening

Group B

1. Roger Federer

Seed: 2

Best five results in 2014: Runner-up at Wimbledon, Winner in Cincinnati and Shanghai, Semi-finalist at Australian Open and US Open

Review: Despite reaching 33, Roger Federer has dismissed any fears that age may be catching up with him with one of his most impressive years of his career. The Swiss has enjoyed better years in terms of results, but he has succeeded in his own battle to remain at the top of the sport.

Reaching the last four of three majors, including the final of Wimbledon, has seen Federer stack up the points, while victories in Dubai, Halle, Cincinnati, Shanghai and Basel has ensured that the former world number one arrives in London as the main threat to Djokovic.

ATP Finals best performance: Winner (2003, 2004, 2006, 2007, 2010, 2011)

Opening match: Raonic on Sunday evening

2. Kei Nishikori

Seed: 4

Best five results in 2014: Runner-up at US Open, Runner-up in Madrid, Winner in Barcelona and Tokyo, Semi-finalist in Paris

Review: Kei Nishikori began 2014 at number 17 in the world rankings but an inspired run on the clay in the spring gave the Japanese star the belief to compete with the sport's very best for the remainder of the year.

A miraculous run to the US Open final saw him come through back-to-back five sets matches before breaking new ground in beating Djokovic, with wins in Kuala Lumpur and Tokyo, as well as reaching the semis in Paris, getting him over the finish line to make his debut at the World Tour Finals.

ATP Finals best performance: Debut

Opening match: Murray on Sunday afternoon

3.Andy Murray

Seed: 5

Best results in 2014: Semi-finalist at Roland Garros, Winner in Shenzhen, Vienna and Valencia, Quarter-finalist in other three majors

Review: After undergoing back surgery late in 2013, the aim for Andy Murray was to slowly adapt back to life on the tour, and that scenario materialised with a number of quarter-finals and semi-finals in the first half of the year.

More last-eight appearances came on the US swing before Murray began to show his best form during an exhausting period which saw him compete six weeks in a row, winning in three of those events, to capture a spot at the tournament when that possibility seemed unlikely after the final major of the year.

ATP Finals best performance: Semi-finals (2008, 2010, 2012)

Opening match: Nishikori on Sunday afternoon

4. Milos Raonic

Seed: 7

Best five results in 2014: Semi-finalist at Wimbledon, Runner-up in Paris, Winner in Washington, Semi-finalist in Rome and Cincinnati

Review: Milos Raonic didn't make much of an impact in 2014 until he made the quarters and semis respectively at Roland Garros and Wimbledon, and that set him for a highly impressive US swing ahead of the US Open.

He couldn't continue that form at Flushing Meadows but final appearances in Tokyo and Paris have given the Canadian the belief that in the conditions facing the players at the O2 Arena, he can defeat anyone put in front of him.

ATP Finals best performance: Debut

Opening match: Federer on Sunday evening