(CNN) Andy Murray this week ended Novak Djokovic's two-year reign atop the rankings in men's tennis but the Scot's stay at No. 1 could be a short one -- and he knows it.

"I might as well try and enjoy it because I could lose it at the tour finals and never be there again," Murray told reporters, referring to the year-end championships, which start Sunday in London.

Indeed, Djokovic can regain the top position on Nov. 21 and finish the season as No. 1 for the third straight time with a strong showing at the World Tour Finals.

The 29-year-old has certainly fared well at the O2 Arena, winning a record four consecutive titles and if he makes it five in a row while winning at least two of his round-robin matches, Djokovic is guaranteed to seize back the No. 1 ranking from Murray.

The last man to qualify for this year's event, Thiem is the youngest player at the finals and the first Austrian to take part in the singles since former world No.1 Thomas Muster in 1997. He may have lost out to Djokovic at Roland Garros -- his first ever grand slam semifinal -- but boasts a 90.9% win percentage in deciding sets in 2016 -- the highest proportion of any player on the tour to have started 25 matches or more. Not bad at all for a 23-year-old. • Aces in 2016: 462 • Win percentage in 2016: 72% • Titles in 2016: 4 - Stuttgart's Mercedes Cup, Open de Nice Cote d'Azur, Acapulco's Abierto Mexicano Telcel, Argentina Open

Dominic Thiem, Austria, No. 8 – The last man to qualify for this year's event, Thiem is the youngest player at the finals and the first Austrian to take part in the singles since former world No.1 Thomas Muster in 1997. He may have lost out to Djokovic at Roland Garros -- his first ever grand slam semifinal -- but boasts a 90.9% win percentage in deciding sets in 2016 -- the highest proportion of any player on the tour to have started 25 matches or more. Not bad at all for a 23-year-old. • Aces in 2016: 462 • Win percentage in 2016: 72% • Titles in 2016: 4 - Stuttgart's Mercedes Cup, Open de Nice Cote d'Azur, Acapulco's Abierto Mexicano Telcel, Argentina Open

For so long looked upon as an entertainer rather than a genuine threat, Monfils is making his ATP World Tour Finals debut in 2016, having produced a career-best season in his 30th year. He reached his first Grand Slam semifinal for eight years at Flushing Meadow, and though he may have eventually lost out to Djokovic, he boasts a higher break point conversion percentage than any other player at the finals. It's been a promising year for the Frenchman; consistency allied with his undeniable talent could see Monfils finally hit the heights he's hinted at. • Titles in 2016: 1 - Washington Citi Open • Aces in 2016: 471 • Win percentage in 2016: 75%

Gael Monfils, France, No. 6 – For so long looked upon as an entertainer rather than a genuine threat, Monfils is making his ATP World Tour Finals debut in 2016, having produced a career-best season in his 30th year. He reached his first Grand Slam semifinal for eight years at Flushing Meadow, and though he may have eventually lost out to Djokovic, he boasts a higher break point conversion percentage than any other player at the finals. It's been a promising year for the Frenchman; consistency allied with his undeniable talent could see Monfils finally hit the heights he's hinted at. • Titles in 2016: 1 - Washington Citi Open • Aces in 2016: 471 • Win percentage in 2016: 75%

Already the first Canadian man to break into the ATP top 10, Raonic became the first to reach a major singles final when he met Murray at Wimbledon. That coveted first Grand Slam title may have evaded him, but he did take the Brisbane International at the turn of the year. Raonic nailed a slam dunk at the 2016 NBA All-Star Celebrity Game, helping Canada beat USA 74-64. And his 6-feet 4-inch stature also helps him on the tennis court ... He wins 90.6% of service games -- the third highest percentage on the tour in the past year (after Ivo Karlovic and John Isner) -- and has the fastest serve (career-best 155.3 miles per hour) of all the ATP World Tour finalists. The Canadian, 25, has also saved an impressive 70.2% of break points -- the highest proportion of the eight. • Titles in 2016: 1 - Brisbane International • Win percentage in 2016: 77% • Aces in 2016: 824

Milos Raonic, Canada, No. 4 – Already the first Canadian man to break into the ATP top 10, Raonic became the first to reach a major singles final when he met Murray at Wimbledon. That coveted first Grand Slam title may have evaded him, but he did take the Brisbane International at the turn of the year. Raonic nailed a slam dunk at the 2016 NBA All-Star Celebrity Game, helping Canada beat USA 74-64. And his 6-feet 4-inch stature also helps him on the tennis court ... He wins 90.6% of service games -- the third highest percentage on the tour in the past year (after Ivo Karlovic and John Isner) -- and has the fastest serve (career-best 155.3 miles per hour) of all the ATP World Tour finalists. The Canadian, 25, has also saved an impressive 70.2% of break points -- the highest proportion of the eight. • Titles in 2016: 1 - Brisbane International • Win percentage in 2016: 77% • Aces in 2016: 824

Djokovic became just the third man in history to hold all four majors at once when he won the French Open in June, and the first player to surpass $100 million in prize money. For now, his throne has been seized by Murray, but it would surely be unwise to write him off; Djokovic is bidding to capture his fifth successive ATP World Finals title, and would move ahead of Pete Sampras and Ivan Lendl in the overall list of winners, equaling Federer with a sixth title. The Serb has never lost to any of his opponents in the Ivan Lendl group -- Gael Monfils (13-0), Dominic Thiem (3-0) and Milos Raonic (7-0) -- and still retains a 71% career win percentage against Murray. • Titles in 2016: 7 - Roland Garros, Australian Open, Canada Masters, Miami Masters, Madrid Masters, Indian Wells, Qatar Open • Aces in 2016: 264 • Win percentage in 2016: 88%

Novak Djokovic, Serbia, No. 2 – Djokovic became just the third man in history to hold all four majors at once when he won the French Open in June, and the first player to surpass $100 million in prize money. For now, his throne has been seized by Murray, but it would surely be unwise to write him off; Djokovic is bidding to capture his fifth successive ATP World Finals title, and would move ahead of Pete Sampras and Ivan Lendl in the overall list of winners, equaling Federer with a sixth title. The Serb has never lost to any of his opponents in the Ivan Lendl group -- Gael Monfils (13-0), Dominic Thiem (3-0) and Milos Raonic (7-0) -- and still retains a 71% career win percentage against Murray. • Titles in 2016: 7 - Roland Garros, Australian Open, Canada Masters, Miami Masters, Madrid Masters, Indian Wells, Qatar Open • Aces in 2016: 264 • Win percentage in 2016: 88%

Standing at 198cm, Cilic is the tallest of the ATP finalists and a full 20cm taller than opponent Nishikori. He reached a third consecutive Wimbledon quarterfinal in 2016, but lost out to Roger Federer despite leading by two sets and holding three match points. Capping a resurgent end to 2016, Cilic has led Croatia to the Davis Cup final with wins over Belgium, USA and France, and will take to the court against Del Potro's Argentina later in November. Murray partly has Cilic to thank for his No. 1 berth -- he upset Djokovic with a supreme display of power tennis in the Paris Masters quarterfinal, beating the Serbian for the first time in his 15th attempt. • Titles in 2016: 2 Cincinnati Masters, Swiss Indoors Basel • Aces in 2016: 636 • Win percentage in 2016: 69%

Marin Cilic, Croatia, No. 7 – Standing at 198cm, Cilic is the tallest of the ATP finalists and a full 20cm taller than opponent Nishikori. He reached a third consecutive Wimbledon quarterfinal in 2016, but lost out to Roger Federer despite leading by two sets and holding three match points. Capping a resurgent end to 2016, Cilic has led Croatia to the Davis Cup final with wins over Belgium, USA and France, and will take to the court against Del Potro's Argentina later in November. Murray partly has Cilic to thank for his No. 1 berth -- he upset Djokovic with a supreme display of power tennis in the Paris Masters quarterfinal, beating the Serbian for the first time in his 15th attempt. • Titles in 2016: 2 Cincinnati Masters, Swiss Indoors Basel • Aces in 2016: 636 • Win percentage in 2016: 69%

The highest-ranked Asian player in ATP history, Kei Nishikori captured a fourth straight title at the Memphis Open in February before going on to win Olympic singles bronze in Rio, beating Nadal in three sets. Nishikori, 26, is the first Japanese singles medalist since Ichiya Kumagae at Antwerp 1920. • Titles in 2016: 1 - Memphis Open • Aces in 2016: 245 • Win percentage in 2016: 76%

Kei Nishikori, Japan, No. 5 – The highest-ranked Asian player in ATP history, Kei Nishikori captured a fourth straight title at the Memphis Open in February before going on to win Olympic singles bronze in Rio, beating Nadal in three sets. Nishikori, 26, is the first Japanese singles medalist since Ichiya Kumagae at Antwerp 1920. • Titles in 2016: 1 - Memphis Open • Aces in 2016: 245 • Win percentage in 2016: 76%

Facing world No.1s in major finals, Wawrinka has never lost -- beating Rafa Nadal in Melbourne (2014), Djokovic at Roland Garros (2015) and the Serb once again at this year's US Open. A man for the big occasion, Wawrinka holds a 100% record against Djokovic in grand slam deciders, but has never otherwise beaten him (0-19). With the US Open title to his name, Wawrinka became the oldest Grand Slam champion (31) since Andre Agassi at the 2003 Australian Open. A remarkable run of winning 11 straight finals only came to an end when he suffered a shock loss in St Petersburg to Alexander Zverev -- citing him as the "future of tennis." • Titles in 2016:US Open, Geneva Open, Dubai Championships, Chennai Open• Aces in 2016:• Win percentage in 2016:

Murray has the highest win percentage on the tour this year and, having followed in the footsteps of Tim Henman (2003) and Greg Rudedski (1998) with victory at the Paris Masters earlier in November, he's now attempting to become the first Brit in history to win the ATP World Tour Finals. A "Group of Death" stands in his way -- with world No.3 Stan Wawrinka having won seven of his 16 matches against Murray in his career. But, in the midst of a 19-match winning streak, Murray won't want to concede the world No.1 spot any time soon. • Titles in 2016: 8 - Wimbledon, Olympics, Queen's Club, Paris Masters, Rome Masters, China Open, Shanghai Masters, Vienna Open • Aces in 2016: 527 • Win percentage in 2016: 89% -- highest of the eight contenders

Andy Murray, Scotland, No. 1 – Murray has the highest win percentage on the tour this year and, having followed in the footsteps of Tim Henman (2003) and Greg Rudedski (1998) with victory at the Paris Masters earlier in November, he's now attempting to become the first Brit in history to win the ATP World Tour Finals. A "Group of Death" stands in his way -- with world No.3 Stan Wawrinka having won seven of his 16 matches against Murray in his career. But, in the midst of a 19-match winning streak, Murray won't want to concede the world No.1 spot any time soon. • Titles in 2016: 8 - Wimbledon, Olympics, Queen's Club, Paris Masters, Rome Masters, China Open, Shanghai Masters, Vienna Open • Aces in 2016: 527 • Win percentage in 2016: 89% -- highest of the eight contenders

Djokovic, however, has struggled since winning the grand slam he craved, the French Open, in June.

A loss of motivation, personal issues and injuries have slowed the Serb, who became the first man in 47 years to claim four straight majors when he defeated Murray in the final at Roland Garros.

His ties to a " spiritual guru " and former Spanish pro, Pepe Imaz, raised eyebrows, especially since his usual coaches, Boris Becker and Marian Vajda, didn't accompany him at the recently concluded Paris Masters, where Djokovic relinquished the No. 1 ranking.

The mantra of Imaz's "tennis group" in Marbella is: "We truly believe love is the way to live life."

Becker and Vajda are expected to return to the Djokovic camp in London and helping Djokovic is that, seemingly, he drew the easier group in the eight-man tournament, having never lost to group rivals Milos Raonic, Gael Monfils and Dominic Thiem. But then Djokovic had never tasted defeat to Marin Cilic in 14 matches -- before losing to the Croat in the City of Light.

The #ATPFinals draw is done ✅ The groups are set ✅ The battle for year-end No1 could come down to the wire @TheO2: https://t.co/PdDuooaFnu pic.twitter.com/RHUleEAb78 — ATP World Tour (@ATPWorldTour) November 7, 2016

Here is a break down of the groups.

John McEnroe group:

Andy Murray. The Scot became a dad this year for the first time, which he says has been instrumental to his success in 2016 as he won a second Wimbledon title and retained his Olympic crown. But it was after the Olympics that he truly stormed to the No. 1 ranking, despite a blip at the US Open. Murray has won 19 straight matches, dropping a mere four sets during the streak. He is bidding to make the final at the year-end championships for the first time.

Murray surely won't be taking his group rivals lightly: Stan Wawrinka has beaten Murray in three of their past four meetings, with Cilic and Kei Nishikori getting the better of Murray in August and September, respectively.

Stan Wawrinka. The Swiss is adamant that he shouldn't be part of any "Big Five" in tennis due to his lack of consistency -- compared, that is, to Djokovic, Murray, Roger Federer and Rafael Nadal. Yet Federer's fellow Swiss continues to prove he is a man for the big occasion, winning a grand slam for three straight seasons. When he toppled Djokovic in the US Open final, he did so from a set down. Wawrinka's lovely -- and bulldozing -- one-handed backhand garners much attention but his forehand is, too, one of the most powerful around.

Kei Nishikori. The Japanese star put in a lackluster performance against Murray at the Olympics but turned it around in New York, outlasting Murray in a pulsating five-setter in the quarterfinals. Injuries continue to hamper Nishikori -- he retired at Wimbledon and then at home in Tokyo -- yet when the Japanese shot-maker is fit and on his game, he possesses the weapons to beat just about anyone. And if matches go to a deciding third or fifth set, his career record is stellar, despite a reverse to Jo-Wilfried Tsonga in Paris: 99-27.

Marin Cilic. The Croat, the 2014 US Open winner -- he beat Nishikori in the final -- is arguably the second most in-form player at the event after Murray. He won the Swiss Indoors over Nishikori and reached the semifinals at the Paris Masters, one day after finally ending his seismic slump against Djokovic. Especially on faster surfaces the 6-feet 6-inch Cilic is a threat: He has a huge serve, impressive two-handed backhand and isn't afraid to play at the net. Cilic's forehand could be the key, though. Prone to wavering, when the forehand is stable it spells trouble for his peers.

He is the latest player at the World Tour Finals to play in the Davis Cup final post London.

Ivan Lendl group:

Novak Djokovic. Can Djokovic replicate his early season performances when he plays at the O2 Arena? His fans hope so. Onlookers might get a feel for Djokovic's form when he challenges Thiem to open the singles on Sunday. Djokovic's much publicized struggles may lead some to wonder if his most dominant stretch of tennis is behind him, although even if the Serb falters at the World Tour Finals, a better gauge would be at January's Australian Open, where he has won the title five times in six seasons.

Thank you #NoleFam for your support! You are amazing!!!! See u in London 👏🏼😁💚 — Novak Djokovic (@DjokerNole) November 7, 2016

Milos Raonic. The Canadian, fourth in aces in 2016, began his campaign with a bang in Melbourne, making the semifinals. Some, too, would argue that if he hadn't sustained an injury he would have ousted Murray after holding a 2-1 lead in sets. Raonic shone again at Wimbledon by advancing to a first grand slam final -- Murray beat him again -- but the injuries have hit him recently. Raonic pulled out of his semifinal against Murray in Paris with a thigh injury and said last week he was unsure of competing in London. Belgium's David Goffin is the first alternate.

Gael Monfils. Ever the showman, Monfils was more focused on court this year -- his . Ever the showman, Monfils was more focused on court this year -- his bizarre antics against Djokovic at the US Open notwithstanding -- and his spot at the year-end championships for the first time is the reward. The Frenchman managed to qualify for London even after missing the French Open due to illness and suffering physically at other stages of the season. Monfils reached the quarterfinals or better at 10 tournaments, highlighted by a semifinal run in New York and final in Monte Carlo.

Dominic Thiem. Just 23, Thiem is the youngest member of the field. His has been a tale of two seasons. Up until the week before Wimbledon the huge-hitting Austrian compiled a 48-12 record, beat the likes of Federer and Nadal and made his first grand slam semifinal at the French Open. But his record from the start of Wimbledon through the Paris Masters slumped to 11-10. Why the dip? The issue for Thiem appears to be playing too much: No one inside the top 20 has played more than Thiem's 27 tournaments this year.