(CNN) -- Outside of the four grand slams, it's the most coveted title in men's tennis.
Through various evolutions, the ATP World Tour Finals is now a $5 million tournament that crowns a grueling, long year.
Bringing together the world's top eight stars in head-to-head competition, it will play to capacity crowds in London's eye-catching O2 Arena, starting on Sunday.
"No easy matches, and terrific tennis," Roger Federer's coach Paul Annacone told CNN. "Definitely the biggest event after the grand slams and met with a lot of anticipation and excitement."
World No.1 Novak Djokovic will seek to top off an amazing season already bulging with three grand slam titles, while defending champion Federer is seeking a record sixth victory in the tournament.
Djokovic is 69-4 for the year and incredibly clinched his place at the finals by mid-May during his 43-match unbeaten start to 2011, but the Serbian has had injury problems of late and may struggle to be at his best.
"I don't feel obliged to win the tournament in order to make this season perfect but I will do my best to prepare and finish off the season in style," he said.
The top eight doubles pairings, led this time by the American Bryan brothers Mike and Bob, will also compete for their end-of-year crown.
Here's CNN's guide to all you need to know about the sport's "fifth major."
Who are the contenders?
Group A: Novak Djokovic, Andy Murray, David Ferrer, Tomas Berdych.
Group B: Rafael Nadal, Roger Federer, Jo-Wilfried Tsonga, Mardy Fish.
Novak Djokovic: Comes into the tournament under an injury cloud after withdrawing from the Paris Masters at the quarterfinal stage with a shoulder injury. But he arrived in London early and has been practicing hard ahead of his opening clash on Monday night against Tomas Berdych. His only previous title at the ATP World Tour Finals came back in 2008 in Shanghai.
Rafael Nadal: The 10-time grand slam champion has never won this title, losing to Roger Federer in last year's final. The Spaniard has played second fiddle to Djokovic for much of 2011, losing his top ranking after the Serb beat him in the finals of Wimbledon and the U.S. Open as well as in four Masters 1000 tournaments. But he did win the French Open on clay for the sixth time and helped Spain reach next month's Davis Cup final against Argentina.
Andy Murray: Home favorite Murray has again missed out in the grand slams in 2011, losing in the final of the Australian Open to the all-conquering Djokovic and losing to Nadal in the semis at the French Open, Wimbledon and the U.S. Open. But he is in hot form after winning three successive events in Asia, including his eighth Masters 1000 title in Shanghai which lifted him above Federer to third in the world rankings.
Roger Federer: The Swiss maestro has been unable to add to his record tally of 16 grand slam titles in 2011, losing in the French Open final after ending Djokovic's winning run in the semis. Came close to repeating the feat at the U.S. Open where he held two match points before losing to Djokovic in the fifth set. But recent titles in Basel and Paris see the 30-year-old in fine fettle as he seeks to make more history.
David Ferrer: The world No. 5's best showing in the finals is a runner-up spot to Federer in the 2007 event in Shanghai. He beat Davis Cup teammate Nadal in January's Australian Open but lost to Murray in the semis. He has reached sixth ATP finals this year, including Masters 1000 tournaments in Monte Carlo and Shanghai, winning titles in Auckland and Acapulco.
Tomas Berdych: The hard-hitting Czech went out at the group stage in his debut last year with a 1-2 record. Best known for reaching the 2010 Wimbledon final, this year Berdych has won the China Open title and beat Murray in November's Paris Masters quarterfinals before losing to Federer.
Jo-Wilfried Tsonga: The popular Frenchman beat Djokovic in his only other appearance in 2008, but still went out in the group stage. Won his sixth ATP Tour title in late September in Metz and comes into the tournament in good form after reaching the final in Paris.
Mardy Fish: The first appearance for the American No. 1, who is renowned for his dashes to the net and fine volleying, but his mobility may be hampered by recent injuries. He won his sixth career title in Atlanta and beat Nadal in the quarters in Cincinnati as he reached a career-high seventh in the rankings.
How did they get there?
Players gain points from playing in the four grand slams, ATP World Tour events including eight Masters Series tournaments, and World Group singles matches in the Davis Cup teams competition. The more prestigious the event, the more points at stake.
The top seven players in the rankings are automatic qualifiers, and the eighth spot is reserved for any grand slam winner not in those places -- a rare occurrence, unless injury strikes.
Alternates are also invited to step in if a players gets injured during the round-robin play. Serbia's Janko Tipsarevic and Spain's Nicolas Almagro fulfill that role this year.
What's at stake?
The ATP World Tour Finals is among the most lucrative on the circuit with an unbeaten winner walking away with a check for $1.63 million.
Players receive $120,000 for just qualifying, with $120,000 for each round-robin win. An undefeated doubles pair pocket $287,000 from an overall prize purse of just over $5 million.
The points gained count towards the final end-of-year ranking. Djokovic is already assured his top spot with a huge gap over Nadal, but back in 2000 Gustavo Kuerten of Brazil dramatically snatched the No. 1 ranking from Marat Safin of Russia when he beat American legend Andre Agassi in the final.
How does it work?
The players are divided into two groups of four and play each other once in a round-robin format, before the top two in each pool advance to the semifinals.
It is possible to lose twice, but still progress and win the tournament. If players are tied on the same number of victories, then the total sets won decides -- if this is also equal then it goes down to total match points.
Matches are over best-of-three tiebreak sets, having changed from best-of-five in 2008.
Can Federer make history?
Tied on five titles with Czech star Ivan Lendl and American legend Pete Sampras, Federer is primed to break the record according to Annacone.
"Roger's been playing well, especially the last couple of weeks, so I think he has a terrific chance," Annacone, who also worked with 14-time grand slam winner Sampras, told CNN.
"The others are also going to be ready and know this is a big opportunity as well, so I think it will be an interesting event in terms of the matches and results."
Where did it all begin?
American Stan Smith was the first winner in Tokyo in 1970, when it was known as the Masters Grand Prix -- organized and promoted by the International Tennis Federation.
A rival tournament, the WCT Finals, was run by the World Championship Tennis Tour and continued to exist in tandem until 1989.
U.S. legend John McEnroe won five WCT titles, as well as three Masters Grand Prix crowns.
The Association of Tennis Professionals (ATP) took over the running of the men's tour from 1990 and the event evolved into the ATP Tour Championship, but the ITF promoted a rival Grand Slam Cup for the top-16 players with the best records in the four majors until 1999.
They reached an agreement to merge the two events and rename it the Tennis Masters Cup, which ran from 2000 until the latest rebranding in 2009 when the tournament moved to the UK capital.
The doubles event, which used to take place at a different venue, has been staged in tandem with the singles since Houston in 2003.