Stefanos Tsitsipas has withdrawn from the ATP Finals due to a right elbow injury and has been replaced by second alternate Cameron Norrie ahead of Wednesday's round-robin matches.
World No. 4 and 2018 ATP Finals champion Tsitsipas was defeated 6-4, 6-4 by Andrey Rublev in his opening match of the tournament in Turin on Monday and the Greek said his elbow had caused him discomfort.
"I have taken the very difficult decision to retire from the 2021 ATP Finals due to my elbow injury, which has been bothering me for a couple of weeks now," Tsitsipas, 23, said in a short statement.
"I was working really hard this year to get to play the Finals and be part of this amazing event but unfortunately I won't be able to continue."
Norrie, who narrowly missed out on automatic qualification for the end of season event, becomes the second injury replacement in the draw after the injured Matteo Berrettini made way for first alternate Jannik Sinner.
Briton Norrie, the world No. 12, will face Casper Ruud on Wednesday, after Novak Djokovic's match against Rublev. Norrie, the Indian Wells champion, will then play Djokovic on Friday.
The 26-year-old becomes the first British singles player to feature at the ATP Finals since Andy Murray won the tournament in 2016.