World No 1 Simona Halep has been forced to withdraw from the season-ending WTA Finals with a lower back injury.
The Romanian, who secured the end of year top ranking for a second successive season earlier this week, has made the difficult decision not to compete in Singapore to put her "long-term health first".
As a result of Halep's withdrawal, Holland's Kiki Bertens will take the eighth and final qualifying spot.
Halep suffered a herniated disc injury while training for the Wuhan Open last month, where she lost in the second round. She then retired midway through her opening match in Shanghai.
The 27-year-old made the decision to compete at the Kremlin Cup in Moscow this week to test the injury but pulled out of the tournament ahead of her match.
Halep, who won a maiden grand slam title at the French Open in the summer said: “Unfortunately, after much discussion with my team and doctors, I have made the decision to withdraw from the WTA Finals in Singapore.
“I wanted to finish 2018 on a high after such an incredible year, but sadly my back injury hasn’t healed the way we hoped it would and I need to put my long-term health first. I’m sad I won’t be able to play the fifth and final edition here in Singapore, but I’m confident this is the right decision and will do everything I can to be back fighting for my place at the WTA Finals next year."
Bertens, who replaces Halep in the top eight line-up, broke into the world top 10 earlier this month.
The 26-year-old had hoped to qualify automatically by reaching the semi-finals of the Kremlin Cup but lost her second-round match to Aliaksandra Sasnovich yesterday.
Her loss saw Karolina Pliskova become the seventh player to qualify, but now Bertens gets the chance to make her second appearance in Singapore, having competed in the doubles field in 2017.
“I’m very happy with my season and to finish it off at the WTA Finals is the ultimate prize,” said Bertens. “I have great memories in Singapore from reaching the doubles final last year. I look forward to being back to compete in front of the amazing crowd.”