World No 2 Simona Halep lasted less than a set before an injury forced her out of the China Open on Sunday, plunging her appearance at the year-ending WTA Finals into doubt.

Halep and Petra Kvitova, who was playing on painkillers and suffering the ongoing effects of glandular fever, both crashed out as the China Open lost its top two women's seeds in the opening round.

Romania's Halep said she would now take a break from tennis and assess if she can play the season finale in Singapore after retiring at 5-4 down in the first set against Lara Arruabarrena.

"My first thought is just to be healthy. Of course, every tournament is important. But the most important thing is to be OK, to feel no pain, and then I will see," Halep said in Beijing.

"I hope I can play in Singapore. That was my goal for this year.

"I need treatment and I need a break, for sure."

Halep, currently at a career rankings high, was understandably downcast after the return of a left Achilles tendon injury which first troubled her in 2011 and flared anew at last month's Guangzhou Open.

Top-ranked Serena Williams is already skipping Singapore as she seeks extra rest after a tough year, while there is a question mark over Maria Sharapova after she retired in Wuhan this week.

Kvitova's push for a place at the eight-player event also hit a snag when she lost 7-5, 6-4 to Sara Errani, hampered by a shoulder injury and the lingering effects of glandular fever.

"I'm kind of sleepy all day long. It's just weird," said the two-time Wimbledon champion, who was diagnosed with the energy-sapping disease earlier this year.

"It's just not a great feeling if you are going for a match. So I don't think it was the case to be motivated. I was coming here really motivated to do well and qualify for Singapore.

"It's (Singapore) still far away right now, so it's kind of disappointing that way."

Elsewhere, soon-to-retire US Open champion Flavia Pennetta battled past China's Han Xinyun 7-5, 4-6, 6-4, and Agnieszka Radwanska reached the second round when Coco Vandeweghe retired.

Along with Vandeweghe and Halep, Lesia Tsurenko also retired as the tail-end of the women's season continued to exact a heavy toll following a number of pullouts last week in Wuhan.