Stefanos Tsitsipas is set to quit the ATP Finals on Tuesday after retiring from his match with Holger Rune early in the first set.
Sixth seed Tsitsipas was trailing 2-1 to Rune when he was forced to quit the Green Group match, handing the Finals debutant his first ever win in the season-ending tournament.
The Greek is now almost certain to withdraw, meaning Hubert Hurkacz will step in to play his final match against Novak Djokovic with no chance of progressing to the semifinals following two defeats.
Tsitsipas had insisted that there was nothing wrong with him physically after Sunday's straight-sets defeat to Jannik Sinner, playing down reports of an elbow injury which curtailed practice ahead of his tournament opener.
Later Djokovic will attempt to reach the semifinals with a match to spare when he takes on home hope Sinner, who is aiming to become the first ever Italian to reach the tournament's last four.
Djokovic secured top spot in the year-end world rankings for a record-extending eighth time with his opening win over Rune and will qualify if he beats Sinner.
That was yet another record in a year which has brought three Grand Slams to take his total to an all-time best 24, as he bids for a seventh Finals title.
Victory in Turin would make him the all-time record winner in the singles, one ahead of old foe and retired great Roger Federer.
Sinner meanwhile will make history if he beats Djokovic in two sets, a tall feat as he has lost all three previous of his encounters with the Serb.