No. 1-ranked Simona Halep has a herniated disk in her back, putting the rest of her season in doubt.
Halep wrote on Twitter on Tuesday that she had an MRI exam that revealed the injury.
She retired from her first-round match at the China Open on Sunday, citing a back problem that has been bothering her.
The season-ending WTA Finals begin in Singapore on Oct. 21.
Halep has lost her past four matches, including at the U.S. Open. She became the first top-seeded woman to lose in the opening round in New York in the 50-year professional era.
The Romanian, who turned 27 last week, is 46-10 with three titles in 2018. Halep won her first Grand Slam championship at the French Open in June, and was the runner-up at the Australian Open in January.