Carlos Alcaraz says he "preferred" to retire from his Paris Masters quarter-final than risk further damage to an abdominal injury before the ATP Finals.
"I cannot stretch. I couldn't serve well, I couldn't hit the forehand well," said US Open champion Alcaraz.
Serbia's Novak Djokovic showed his all-round quality as he beat Italy's Lorenzo Musetti to reach the last four.
Reigning champion Djokovic, going for a record-extending seventh title, produced what he described as one of his "best matches of the year" to win 6-0 6-3 against 20-year-old Musetti at the indoor event in Paris.
The 35-year-old was already assured of appearing in the ATP Finals because of his status as Wimbledon champion, but this victory ensured he would have qualified as one of the year's top eight players anyway.