Novak Djokovic (Serbia)
World ranking: 1
2018 titles: Wimbledon, Cincinnati Masters, US Open, Shanghai Masters
ATP Finals record: 10 appearances (champion 2008, 2012, 2013, 2014, 2015)
A season that was shaping up as one to forget has dramatically transformed into one of major milestones and stunning success. A problematic elbow cut short Djokovic's 2017 season in July and a premature return at January's Australian Open proved ill-advised.
After taking two months off for elbow surgery, the Serbian struggled for form, suffering early exits at Indian Wells, Miami, Monte Carlo, Barcelona and Madrid.
Gradually Djokovic found his rhythm, and his march to the Queen's final in London in June was the turning point. From there he embarked on a sparkling run, collecting the Wimbledon title and the Cincinnati Masters - becoming the first player to win all nine Masters - before clinching his 14th major at the US Open.
The Shanghai Masters soon followed as did a run to the Paris Masters final and a return to world No 1.
Far and away the best player in the world right now, 31-year-old Djokovic will look to complete a remarkable second half of the season with his sixth ATP Finals title.
Roger Federer (Switzerland)
World ranking: 3
2018 titles: Australian Open, Rotterdam Open, Stuttgart Open, Basel Indoor
ATP Finals record: 15 appearances (champion 2003, 2004, 2006, 2007, 2010, 2011)
Another selective season for the Swiss started in flawless fashion, winning his sixth Australian Open title - and 20th grand slam - in January before going to Rotterdam a month later and collecting the title and going to No 1 for the first time in five years.
A nail-biting defeat to Juan Martin del Potro in the Indian Wells final followed before a shock defeat in the opening round in Miami. Federer bounced back to win the Stuttgart Open to kickstart his grass court season but then lost to Borna Coric in the Halle final.
The second half of the season has been somewhat disappointing for Federer, with defeat to Kevin Anderson in a Wimbledon quarter-final marathon and a comprehensive loss to Djokovic in the Cincinnati final. Then there was the shock defeat to world No 55 John Millman in the US Open fourth round and another loss to Coric in Shanghai.
There have been signs in recent weeks that Federer is rediscovering his form, winning the Basel Indoor before narrowly losing to Djokovic in the Paris semi-finals.
Set for his 16th ATP Finals appearance, Federer, 37, will need to continue his recent good form to be a contender in London.
Alexander Zverev (Germany)
World ranking: 5
2018 titles: Munich Open, Madrid Masters, Washington Open
ATP Finals record: 1 appearance (group stage 2017)
Overall, it has been another impressive year for Zverev: three more titles to add to his growing collection, including one of the standout tournament performances of the season by winning the Madrid Masters without facing a break point all week.
The 21-year-old German also came close in Miami, losing a tense final to John Isner, before giving Rafael Nadal an almighty scare in the Rome final before going down to the Spaniard.
However, it has been another year of curious underachievement in the four grand slams. A third round loss at the Australian Open was followed by his first quarter-final appearance at a major at the French Open, although Zverev required three successive five-set wins to reach the last eight. He was then defeated in the third round at both Wimbledon and the US Open.
Undoubtedly a serial grand slam champion in the making, Zverev should be a leading contender at the ATP Finals, so long as his shoulder injury heals in time.
Kevin Anderson (South Africa)
World ranking: 6
2018 titles: New York Open, Vienna Open
ATP Finals record: Debut
After narrowly missing out on a place at the ATP Finals in 2017, Anderson will be the fourth-highest ranked player competing this year, proving the 32-year-old South African continues to get better with age.
Given his form over the past two years, it's easy to forget that Anderson was a perennial top-20 player for much of his career, his rise to elite status even more remarkable having returned from a knee injury that saw him slump to world No 80 in 2016.
This year has yielded two more titles, winning the inaugural New York Open (previously the Memphis Open) in February, and more recently the Vienna Open last month.
Anderson also reached his second grand slam final, making the Wimbledon showpiece after an epic semi-final against John Isner the South African won 26-24 in the final set. Unsurprisingly, Anderson struggled against Djokovic in the final and went down in straight sets.
Anderson enters the ATP Finals confident and in good form, and will fancy his own chances in a group that should be tightly-contested.
Marin Cilic (Croatia)
World ranking: 7
2018 titles: Queen's Club
ATP Finals record: 3 appearances (group stage 2014, 2015, 2017)
The 2018 season has been somewhat emblematic of Cilic as a player: areas of brilliance mixed with spells of mediocrity. When on song, few players can hang with the 30-year-old Croatian as he proved in his run to the Australian Open final where, at one point, he had Federer on the ropes.
There was also Cilic's victorious campaign at Queen's as he outlasted Djokovic to clinch the title.
Cilic also enjoyed some relative success during the clay court swing on what is undoubtedly his weakest surface, reaching the Roland Garros quarter-finals and the Rome semi-finals.
Yet there have been alarming drops off in form as well. Cilic looked frighteningly good against Guido Pella in the Wimbledon second round, cruising to a 2-0 lead, before inexplicably losing in five sets. Then there has been his recent run of five losses in seven matches, halted by a close defeat to Djokovic in Paris.
Cilic's hopes at the ATP Finals will hinge on which Cilic shows up in London. If we are treated to the player who won Queen's, then he is a serious contender, but if it's the player who stumbled to defeat against then world No 93 Maruis Copil in Basel, it will be another group stage exit.
Dominic Thiem (Austria)
World ranking: 8
2018 titles: Buenos Aires Open, Lyon Open, St Petersburg Open
ATP Finals record: 2 appearances (group stage 2016, 2017)
Another year that has seen Thiem solidify his status as one of the leading clay court players, but the Austrian has also made strides on other surfaces.
He collected his first title in February at the Buenos Aires Open, before struggling in the American hard court swing and suffering a 6-0, 6-2 defeat to Nadal in the Monte Carlo quarter-finals.
Thiem bounced back to reach the Madrid Masters final but was outplayed by Zverev, and weeks later clinched title No 2 in Lyon. That set Thiem up for his biggest milestone of the season when he reached his first grand slam final at Roland Garros where he lost to Nadal.
Thiem, 25, has shown good form during the latter part of the season, winning the St Petersburg title on an indoor hard court before reaching his first non-clay Masters semi-final in Paris last time out.
He is in good form but will have to play his best tennis to have any chance of progressing from his group.
Kei Nishikori (Japan)
World ranking: 9
2018 titles: 0 (Finalist in Vienna, Tokyo, Monte Carlo)
ATP Finals record: 3 appearances (semi-finals 2014, 2016)
Nine is the key number for Nishikori this season. Having fallen to No 39 in the world rankings as recently as April, the Japanese No 1 is back to No 9 and earned his place at the ATP Finals as the first reserve when Nadal and Del Potro ruled themselves out with injury.
After recovering from wrist injury, Nishikori has enjoyed a number of deep runs throughout the season, starting with a final appearance in Monte Carlo in April.
Results were mixed for a spell during the summer, which included an impressive march to the Wimbledon quarter-finals but early defeats at the Rogers Cup and Cincinnati.
The latter part of the season has seen greater consistency, starting with a semi-final at the US Open. Nishikori then reached successive finals, in Tokyo and Vienna, but the 28-year-old was unable to snap his losing final streak: he has now lost nine straight finals.
Nishikori, a two-time semi-finalist at the ATP Finals, has good history in London and is producing his best tennis of the season at just the right time. He could well be the tournament dark horse.
John Isner (United States)
World ranking: 10
2018 titles: Miami Open, Atlanta Open
ATP Finals record: Debut
In many ways, 2018 has been the stand-out season of Isner's career. The big-serving American won his first Masters title, in Miami, and reached his first grand slam semi-final at Wimbledon.
The season started woefully though, with seven defeats in his first eight matches - six of which were to players ranked outside the top 50. Then seemingly out of nowhere, Isner caught fire to win the Miami title.
He understandably struggled during the clay court swing, but matched his best result at the French Open by reaching the fourth round.
Next up was his run to the Wimbledon semi-finals where he narrowly lost an epic to Anderson, before carrying his form to Atlanta where he won the title.
Isner also matched his best effort at the US Open by reaching the quarter-finals but his form has been sporadic since.
He is rightly considered an outsider at the ATP Finals, but if he can get that monster serve working, Isner can cause problems for any opponent.