An emotional Matteo Berrettini withdrew from the ATP Finals after failing to recover from an abdominal injury that forced him to retire from his opener.
The Italian had been due to face Hubert Hurkacz in his second group-stage match in Turin on Tuesday (20:00 BST) but pulled out four hours before it.
"I thought, thought, cried and finally decided... my finals end here, I'm devastated," he posted on Instagram.
He will be replaced by compatriot Jannik Sinner, who is first alternate.
Britain's Cameron Norrie is the second alternate at the season-ending event for the year's leading eight men.