Simona Halep could be a doubt for the Tour Finals after withdrawing from the WTA event in Moscow with an ongoing back injury.
The Romanian, who will end the year as the world number one regardless of her involvement in Singapore, has struggled with the problem since losing in the Cincinnati final to Kiki Bertens.
Halep had taken a wildcard to enter the event in the Russian capital but has opted to skip the tournament without taking to the court, meaning Valentini Grammatikopoulou will compete as a lucky loser in the second round and will face Anastasia Pavlyuchenkova.
Halep reached the final of the season-ending showpiece in her tournament debut four years ago but has failed to pass the group stage in her three appearances since.
The WTA Finals is due to begin on Sunday and Halep’s withdrawal from Moscow is probably a precautionary measure, but it still raises big concerns over the 27-year-old’s fitness ahead of the event.
If fit, Halep will be joined by Angelique Kerber, Caroline Wozniacki, Naomi Osaka, Petra Kvitova and Sloane Stephens in Kallang, with two spots still up for grabs.
Eilina Svitolina, Karolina Pliskova and Bertens are in a three-way fight for the final two places in Singapore.
Halep ended her wait for a Grand Slam crown at the French Open earlier this year and she is 5.00 to successfully defend her title in Paris in 2019, while she can be backed at 9.50 to land the Australian Open title at the start of next season.