Britain's Cameron Norrie will make his debut at the ATP Finals after Stefanos Tsitsipas withdrew injured from the event in Turin.
The 26-year-old was second alternate for the season-ending tournament with first reserve Jannik Sinner having replaced Matteo Berretini on Tuesday.
Norrie will take on Norway's Casper Ruud on Wednesday (20:00 GMT) and world number one Novak Djokovic on Friday.
The ATP said Greece's Tsitsipas had withdrawn with a right elbow injury.
Playing at the elite tournament, which features the year's eight leading players, caps a successful year for Norrie, who won the prestigious Indian Wells tournament and became British number one in October.