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
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
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
Nishikori was given the call when Juan Martin del Potro pulled out due to a knee injury. The Japanese will be playing in the tournament for a fourth time after an encouraging season, though he has not won a tournament since 2016.
World ranking: 9
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