Novak Djokovic and Roger Federer have 11 ATP Finals titles between them and they are the two standout contenders in London.
There will be no Rafael Nadal in London, but Roger Federer will go in search of a century of titles and Novak Djokovic will take some beating in the ATP Finals.
Nadal withdrew from the season-ending tournament - which starts on Sunday - earlier this week due to a persistent abdominal injury and then underwent ankle surgery after pulling out.
Djokovic replaced Nadal as world number one on Monday and is the top seed at an event he has won five times.
Six of Federer's 99 titles have come at the ATP Finals and he would raise the roof at the O2 Arena by reaching three figures.
We take a look at the eight contenders for the singles title, including debutants Kevin Anderson and John Isner.
1. Novak Djokovic
Nadal's withdrawal ensured Djokovic is guaranteed to end the year at the top of the rankings for a fifth time following an incredible resurgence, having undergone elbow surgery in February.
World ranking: 1
Titles in 2018: Four (Wimbledon, Cincinnati Masters, US Open, Shanghai Masters)
Last five tournaments (most recent first): Paris Masters - runner-up, Shanghai Masters - champion, US Open - champion, Cincinnati Masters - champion, Rogers Cup - last 16
ATP Finals history: 2016 - runner-up, 2015 - champion, 2014 - champion, 2013 - champion, 2012 - champion, 2011 - round-robin stage, 2010 - semi-finals, 2009 - round-robin stage, 2008 - champion, 2007 - round-robin stage
2. Roger Federer
The legendary Swiss started another fruitful year by claiming his 20th grand slam title at the Australian Open and moved on to 99 tournament triumphs with a Swiss Indoors Basel success in his hometown last month.
World ranking: 3
Titles in 2018: Four (Australian Open, Rotterdam Open, Stuttgart Open, Swiss Indoors Basel)
Last five tournaments: Paris Masters - semi-finals, Swiss Indoors Basel - champion, Shanghai Masters - semi-finals, US Open - last 16, Cincinnati Masters - runner-up
ATP Finals history: 2017 - semi-finals, 2015 - runner-up, 2014 - runner-up, 2013 - semi-finals, 2012 - runner-up, 2011 - champion, 2010 - champion, 2009 - semi-finals, 2008 - round-robin stage, 2007 - champion, 2006 - champion, 2005 - runner-up, 2004 - champion, 2003 - champion, 2002 - semi-finals
3. Alexander Zverev
Zverev has failed to do himself justice in grand slams again this year, but the 21-year-old has added another three titles to his list of honours and has 54 victories to his name in 2018 - more than any other player.
World ranking: 5
Titles in 2018: Three (BMW Open, Madrid Masters, Citi Open)
Last five tournaments: Paris Masters - quarter-finals, Swiss Indoors Basel - semi-finals, Shanghai Masters - semi-finals, China Open - last 16, US Open - last 32
ATP Finals history: 2017 - round-robin stage
4. Kevin Anderson
Anderson will make his debut in London following another strong season, which included a second grand slam final appearance at Wimbledon, where Djokovic outclassed him in the decider.
World ranking: 6
Titles in 2018: Two (New York Open, Vienna Open)
Last five tournaments: Paris Masters - last 16, Vienna Open - champion, Shanghai Masters - quarter-finals, Japan Open - quarter-finals, US Open - last 16
ATP Finals history: Debutant
5. Marin Cilic
Cilic has won just the one title this year and will be looking to address a poor record in the season-ending showpiece, having won only one of his nine matches.
World ranking: 7
Titles in 2018: One (Aegon Championships)
Last five tournaments: Paris Masters - quarter-finals, Swiss Indoors Basel - last 16, Shanghai Masters - last 32, Japan Open - last 32, US Open - quarter-finals
ATP Finals history: 2017 - round-robin stage, 2016 - round-robin stage, 2014 - round-robin stage
6. Dominic Thiem
Thiem reached his first major final at the French Open in June, missing out on the biggest prize of his career as Nadal produced another masterclass on clay. Only Zverev has won more matches than the Austrian this year.
World ranking: 8
Titles in 2018: Three (Argentina Open, Lyon Open, St. Petersburg Open)
Last five tournaments: Paris Masters - semi-finals, Vienna Open - quarter-finals, Shanghai Masters - last 32, St. Petersburg Open - champion, US Open - quarter-finals
ATP Finals history: 2017 - round-robin stage, 2016 - round-robin stage
7. Kei Nishikori
Nishikori did not qualify automatically, but got the call to extend an encouraging season when Juan Martín del Potro pulled out due to a knee injury.
World ranking: 9th
Titles in 2018: None
Last five tournaments: Paris Masters - quarter-finals, Vienna Open - runner-up, Shanghai Masters - quarter-finals, Japan Open - runner-up, Moselle Open - semi-finals
ATP Finals history: 2016 - semi-finals, 2015 - round-robin stage, 2014 - semi-finals
8. John Isner
Isner is another debutant at the O2 Arena after he replaced Nadal, the big-serving American prolonging a season which brought him the biggest title of his career on home soil in Miami.
World ranking: 10
Titles in 2018: Two (Miami Open, Atlanta Open)
Last five tournaments: Paris Masters - last 16, Vienna Open - last 16, Stockholm Open - semi-finals, US Open - quarter-finals, Cincinnati Masters - last 64
ATP Finals history: Debutant