World number one Simona Halep has withdrawn from next week's WTA Finals because of a lower back injury.

The Romanian sustained the injury - a herniated disc - during a training session before last month's Wuhan Open.

She said she not ready to compete at this level and didn't practise for the last four weeks.

World number 10 Kiki Bertens takes Halep's place in the event, where $7m (£5.34m) prize money is available.

Halep said it's always difficult to withdraw from a big tournament and it's the first time she had to take a tough decision but it's better for her health.

The French Open champion received treatment on the back problem during her defeat by Dominika Cibulkova in the second round of the Wuhan Open and then withdrew when one set down to Tunisia's Ons Jabeur in the first round of the China Open.