Daniil Medvedev, who won the US Open in 2021, is the defending ATP Finals champion having won the title in 2020.
The 20-year-old Jannik Sinner is in with an outside chance of making the ATP Finals. But his 2021 season has been excellent regardless.
David Nalbandian became the first player to win the year-ending ATP Final title without winning a Grand Slam title.
Roger Federer and Novak Djokovic have dominated the ATP Finals title but Rafael Nadal has never won it.