SINGAPORE - World No.1 Simona Halep has been forced to withdraw from the BNP Paribas WTA Finals Singapore presented by SC Global due to an ongoing back injury.

Halep sustained the injury during a pre-tournament practice session in late September at the Dongfeng Motor Wuhan Open, which was later diagnosed as a disk hernia. Having flowing to Singapore in hopes her injury might subside, Halep confirmed her withdrawal on Thursday.

"The decision was tough for sure," Halep told WTA Insider. "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. 

"Darren [Cahill, her coach] was 100% to withdraw because he's worried about the back and it's better to put the health first, always. He always did that with me since we started. So he was more sure than me that we had to pull out. Then when I heard that he agreed with this it was easier for me to take the decision."

"My back is not ok. 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 has enjoyed the best season of her career in 2018, which saw her begin the season with an astounding physical effort to make her first Australian Open final, which set the stage for her maiden Slam title at Roland Garros in the spring. Then came another pounding two weeks in North America over the summer, where she won her third title of the season at the Coupe Rogers in Montreal and made the final at the Western & Southern Open.

Since that summer run, the 27-year-old has been forced to battle her body. Halep has not played a match since her injury-addled 6-0, 7-5 loss to Dominika Cibulkova in the opening round of Wuhan. She went on to retire to Ons Jabeur in the first round of the China Open and withdrew from the Kremlin Cup earlier this week. 

"It's the same back problem from Wuhan and I need a break to heal it," Halep said. "I need three to four weeks doing exercises and having a break and it should be fine. So I have enough time now to get ready for the off-season and to get ready for the next year."

Since Singapore became the WTA Finals host city in 2014, the Romanian is the only woman to have qualified all five years. The Lion City is where Halep earned one of the biggest wins of her career, a 6-0, 6-2 win against Serena Williams in round-robin play in 2014, it is the first tournament she played as a newly-minted WTA World No.1 last year, and it is where she was crowned the 13th WTA Year-End No.1, a title she has claimed once again this season. 

A finalist in Singapore's inaugural staging, Halep's strong connection and commitment to the WTA's crown jewel event made her decision even tougher. 

"This tournament is one of the most important because it is only eight players and it shows how good you worked this year," Halep said. "So it's really important and it meant so much to me because I qualified five years in a row. I'm feeling sad that I can't compete but sometimes it happens. This tournament, because of the format, because you play every match as a final, it's really tough to compete here and you have to be 100% every time."

Set to be the top seed at her fifth consecutive WTA Finals, Halep's withdrawal moves the Netherlands' Kiki Bertens, who is No.9 on the Porsche Race to Singapore, into the field. The World No.10, who won three titles this season, including a win over Halep to capture the biggest title of her career in Cincinnati, will be making her tournament debut. She will be the first Dutchwoman to compete at the WTA Finals in over 20 years, since Brenda Schultz-McCarthy in 1997.

With Halep's withdrawal, the Elite Eight Singapore field in the order they qualified: 

1. Angelique Kerber
2. Naomi Osaka
3. Petra Kvitova
4. Caroline Wozniacki
5. Sloane Stephens
6. Elina Svitolina
7. Karolina Pliskova
8. Kiki Bertens

The draw ceremony for the BNP Paribas WTA Finals Singapore presented by SC Global will be held on Friday, October 19th. Round Robin play begins on Sunday, October 21st.