World No. 1 Simona Halep has qualified for all five editions of the BNP Paribas WTA Finals Singapore presented by SC Global, but she will not compete at this year's competition that starts on Sunday.
She withdrew from the US$7 million (S$9.65 million) season-ender owing to a lower back injury.
The French Open champion, who is already in Singapore, said in a statement yesterday: "Unfortunately, after much discussion with my team and doctors, I have made the decision to withdraw.
"I wanted to finish 2018 on a high after such an incredible year. 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 in Singapore, but I'm confident this is the right decision and I will do everything I can to be back fighting for my place at the Finals next year.
"Singapore is a special city and I'm sure the fans will enjoy seeing some fantastic women's tennis this week."
The 27-year-old Romanian, who won her maiden Grand Slam title at Roland Garros this year, had pulled out from this week's Kremlin Cup in Moscow for the same reason.
Replacing her in the Singapore singles line-up is Kiki Bertens, a losing finalist in the doubles with Sweden's Johanna Larsson last year.
Said the 26-year-old Dutchwoman: "I'm very happy with my season and to finish it off at the WTA Finals is the ultimate prize.
"I have great memories from reaching the doubles final. I look forward to being back to compete in front of the amazing crowd."
The world No. 10 had defeated Halep in August to win the biggest title of her career at the Western and Southern Open in Cincinnati. She also became the first woman from her country to break into the top 10 in 22 years.
After Serena Williams' absence, as the top qualifier in 2015 and then the No. 2 in 2016, this is the third time a player in the top two has withdrawn from the Singapore event.
World No. 3 Angelique Kerber does not believe Halep's absence will make it less competitive, saying: "All the players (who are) here deserve to be here - all have had a consistent year and these are the best players of (this season). It doesn't matter who you're playing, you have to play your best."
Kerber, Caroline Wozniacki, Naomi Osaka, Petra Kvitova, Sloane Stephens, Elina Svitolina and Karolina Pliskova complete the field.