World number one Serena Williams will fly to Europe for a scan on the knee injury that forced her out of the China Open ahead of her quarter-final.
It is the second tournament in a row the 33-year-old has pulled out of, after she fell ill in her first set at the Wuhan Open in China last week.
"It throbs just sitting, standing. I feel it mostly serving because I'm landing on my left knee," she said.
"I haven't had time for an MRI but I'll do that and see what the problem is."
|Serena Williams on battling injury in Beijing|
|"As a defending champ, you always want to do your best to defend your title. That's why I played on Thursday - I had to at least give it a chance."|
Her last-eight opponent Sam Stosur will now go straight into the semi-finals in Beijing, where she will face Wimbledon champion Petra Kvitova.
Maria Sharapova, a 6-0 6-4 winner over fellow Russian Svetlana Kuznetsova, faces Ana Ivanovic in the other semi-final.
Williams won her 18th Grand Slam at the US Open last month, putting her joint-fourth in the all-time list of major winners, alongside Martina Navratilova and Chris Evert.
But last week, in her first tournament since Flushing Meadows - the inaugural Wuhan Open - she had to be helped from the court during her match against Alize Cornet.
At this year's Wimbledon, the world number one withdrew from the doubles tournament with sister Venus because of a viral illness.
|Serena Williams in 2014|
|Six titles: Brisbane International (January), Sony Open (March), Italian Open (May), Bank of the West Classic (August), Western & Southern Open (August), US Open (September). Singles record: 45 wins, 7 defeats; Doubles record: 4 wins, 3 defeats|
Her knee became inflamed late on Thursday after she won her third round match 6-1 1-6 6-2 against Czech Lucie Safarova on the Beijing hard courts.
"I started to feel pain this week," she told a news conference. "It didn't feel really well last night at all. I almost didn't go on the court [against Cornet] - I did at the last minute. I think it made it a little worse, to be honest.
"As a defending champ, you always want to do your best to defend your title. That's why I played yesterday [Thursday]. I had to at least give it a chance."
The WTA Finals begin in Singapore on 17 October and she added: "If I feel this way for Singapore, I don't think it's smart for me to play. I'm just going to see how it goes."
Older sister Venus also withdrew from the China Open, citing a viral illness.
World number three Kvitova said after her 7-6 (7-2) 6-4 win over Italian Roberta Vinci that she was not surprised the long season was taking a toll.
"It's really difficult. I mean, I'm not feeling full of energy. I'm tired," said the Czech, who won the Wuhan title last week. "It's tough to find energy in the locker room."