Halep was set to face Veronika Kudermetova in the second round. Kudermetova received a walkover into the Round of 16 and will face either Paula Badosa or Ajla Tomljanovic.
World number one Simona Halep withdrew from the WTA Finals in Singapore with a back injury on Thursday, bringing her season to a premature end.
The French Open champion pulled out of last month’s China Open after just 31 minutes, and withdrew from this week’s Kremlin Cup because of the persistent problem.
Halep, 27, said she had hoped to compete at the season finale in Singapore, which starts on Sunday, but that she had taken the “tough” decision to put her health first.
“It’s always difficult to withdraw from a big tournament,” said the Romanian, who sustained the injury in Wuhan last month.
“My back is not okay. I didn’t practise for the last four weeks. I’m not ready to compete at this level and I’m still sore.”
Her withdrawal means that Kiki Bertens from the Netherlands moves into the eighth and final qualifying spot in the WTA Finals.
It has been a disappointing end to a spectacular season for Halep, which saw her secure the year-end number one ranking in the WTA standings for the second year running.
She spent 40 weeks at number one in 2018, and won three titles from a tour-leading six final appearances, amassing a 46-11 win-loss record.
This year Halep has won the Shenzhen Open, the Rogers Cup and her debut Grand Slam crown at Roland Garros.
In addition, she reached the title match at the Australian Open, in Rome and Cincinnati.