Stefanos Tsitsipas retired because of injury after three games of his ATP Finals match against Denmark’s Holger Rune.
Tsitsipas, 25, trailed 2-1 in his Green Group match and after consulting with a doctor at the changeover pulled out.
The Greek world number six appeared to be troubled by a back problem and left the court to a few jeers from the crowd inside Pala Alpitour in Turin, Italy.
His withdrawal from the match means he cannot progress to the semi-finals.
Novak Djokovic is now able to book his spot in the last four with a win over Jannik Sinner later on Tuesday.
If Tsitsipas officially withdraws from the ATP Finals, then number one alternate Hubert Hurkacz will replace him for his final match against Djokovic.
