Defending champion Novak Djokovic has pulled out of the ATP Finals because of an "ongoing injury".
The 37-year-old Serb, who has won a record seven titles at the event, had already qualified as the sixth best player on the ATP Tour this season.
"I was really looking forward to being there, but due to ongoing injury I won't be playing next week," Djokovic wrote on social media.
"Apologies to those who were planning to see me. Wishing all the players a great tournament. See you soon."
Djokovic, who has won a record 24 Grand Slam men's singles titles, indicated last month that competing in the tour's year-end finals was not a priority.