World number one Simona Halep has withdrawn from the upcoming WTA Finals in Singapore over back problems which means that Dutch player Kiki Bertens takes the final place at the season-ender.

The WTA said Thursday that Halep suffered a disk hernia during training last month. She nonetheless flew to Singapore hoping to compete at the October 21-28 event but eventually had to quit.

"The decision was tough for sure," Halep told the WTA website. "It's always difficult to withdraw from a big tournament. Actually, I think it's the first time in my life. So it was a tough decision but it's better for my health.

"My back is not okay. I didn't practice for the last four weeks. I'm not ready to compete at this level and I'm still sore. It's not easy to go on court and compete at this moment."

Halep won her first career grand slam title this year at the French Open and will end the year as the top-ranked player despite not being able to play in Singapore.

Her withdrawal sent Bertens to the eight-player finale for the first time, despite an early exit Wednesday at the Kremlin Cup in Moscow.

The other qualifiers are Denmark's Caroline Wozniacki, Angelique Kerber of Germany, Japanese Naomi Osaka, American Sloane Stephens, Elina Svitolina of Ukraine and Czechs Petra Kvitova and Karolina Pliskova.