The world number one's place at the WTA Finals is in doubt after she was forced to withdraw from the Kremlin Cup.
French Open champion Simona Halep has pulled out of the Kremlin Cup with a back injury, casting further doubt on her participation in the WTA Finals.
The world number one recently revealed that she had been diagnosed with a herniated disc after struggling with an issue at the Wuhan Open and then the China Open.
Halep offered an update on Monday, as she was named the year-end number one for the second successive year, admitting she had not yet trained "100 per cent" just 48 hours before her scheduled Moscow opener against Anastasia Pavlyuchenkova.
And the Romanian has now confirmed that she has had to withdraw from the tournament, with her place in Singapore uncertain given she stated it would be "very doubtful" she could miss the Kremlin Cup and then feature in the Finals.
I really wanted to play here in Moscow, but unfortunately my back is still causing me pain and I don't want to take any unnecessary risks.— Simona Halep (@Simona_Halep) October 16, 2018
While it's disappointing to withdraw, I know it's important to put my health first. I wish the tournament great success pic.twitter.com/vqGiclchyD
Posting on her Twitter page, Halep wrote: "I really wanted to play here in Moscow, but unfortunately my back is still causing me pain and I don't want to take any unnecessary risks.
"While it's disappointing to withdraw, I know it's important to put my health first. I wish the tournament great success."
The Finals begin next week, with the qualifying berths finalised in Moscow.