World number one Serena Williams is currently shattered as she plans to 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 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.