Sixth seed Pegula withdraws from WTA Finals due to knee injury
A runner-up at the WTA Finals last year, Pegula lost her opening two matches in Saudi Arabia in straight sets and had no chance of advancing to the semi-finals.
The US Open finalist was due to face defending champion Iga Swiatek in the last day of round-robin action on Thursday and will now be replaced by Daria Kasatkina on the schedule.
"I've just been struggling with a little bit of an injury and for whatever reason, it really flared up this week. Not really sure why," explained the 30-year-old.
"In the few days before practice, it felt really, really good, but it started creeping up right before my first match, and just feel like I'm not feeling comfortable moving on court, so I just wanted to make the decision to unfortunately pull out."
Kasatkina is ranked number nine in the world and recently clinched the WTA 500 title in Ningbo.
She is 1-5 head-to-head against Swiatek, losing her last five consecutive clashes with the Pole.
Coco Gauff is the only player in the Orange Group in Riyadh to have secured passage to the semi-finals.
