Stefanos Tsitsipas is out of the ATP Finals and Cameron Norrie has replaced him in the elite eight-man field.
A lingering right elbow injury has knocked Tsitsipas out of Turin.
More: Berrettini Ends Season in Tears
The injury forced Tsitsipas to retire from the Rolex Paris Masters earlier this month. Tsitsipas played through the pain in his ATP Finals round-robin opening loss to Andrey Rublev, but announced his withdrawal today.
“I’ve taken the 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 said. "It’s a very difficult decision from my side.
"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."
Indian Wells champion Norrie will take on Casper Ruud in tonight's Green Group round-robin match.
It's the second time in as many days a player has withdrawn. Italian Jannik Sinner stepped in for injured compatriot Matteo Berrettini and beat Hubert Hurkacz yesterday.
Photo credit: ©Nicolas Gouhier/FFT/Rolex Paris Masters