Serena Williams held a closed indoor training session at the Australian Open on Wednesday, increasing speculation about her fitness after she painfully rolled her ankle on court.
The world number three and hot tournament favourite twisted her right ankle during a 6-0, 6-0 demolition of Romania's Edina Gallovits-Hall in her opening round clash on Tuesday.
The defiant 15-time Grand Slam champion claimed only a "fatal" injury would prevent her contesting the second round against Spain's Garbine Muguruza on Thursday.
But the reigning Wimbledon, Olympic and US Open champion moved her scheduled session from court 17 to the indoor training facility at Melbourne Park and it was closed to the media and public, tennis officials said.
Hundreds of spectators were ten-deep around court 17 until it became clear that the five-time Australian champion would not be making an appearance. Instead, she was hitting balls indoors.
Williams did not appear to be in any discomfort when she was seen walking through Melbourne Park by an AFP reporter with her coach, Patrick Mouratoglou.
Leading 4-0 against Gallovits-Hall, Williams fell heavily and lay on her back for an extended time, her face in her hands.
Despite this, she returned to defeat the world number 110 in 54 minutes, losing only six points in the second set.
"That's definitely optional. I'm going to play it by ear," Williams said after her match.
"I would love to see the next few hours how I go, and then I'm going to decide what to do next at that stage."
"Obviously there's pain, obviously there's swelling. So it's going to be really important to see how the next few hours unfold."
Williams is the reigning Wimbledon and US Open champion and has won the Australian title five times.
Meanwhile, fellow luckless American Brian Baker was struck down by a fresh injury nightmare at the Australian Open on Wednesday, after battling back from six years on the sidelines.
The world number 57, who bravely returned to tennis in 2011 following a series of operations, was a set down against compatriot Sam Querrey when he collapsed clutching his knee.
"It didn't look very good," said Querrey. "He said he kind of just felt his knee almost buckle and kind of heard like a pop or a snap.
"He didn't know if it was bones or a tear, but he couldn't straighten it, couldn't walk. I feel awful for him."
Baker, 27, was whisked away for treatment and was not immediately available for interview.
He stunned former French Open winner Gaston Gaudio in the first round of the 2005 US Open but has needed five operations in all for hip, elbow and hernia problems.
"He's the last person that deserves anything like that," Querrey said. "He does everything right, treats his body great, just trying to come back, and then something like that happens, it's just so unlucky.
"He was just going, 'I just can't believe it'. He just had his head down, just like, 'Why is this happening to me?'"
Baker, who reached his first career final in Nice last year, and also played the French Open and Wimbledon, earlier explained to the ATP website why he decided to return to the tour after so long away.
"I felt like I had some unfinished business. My body was the main factor why I took off. I always wanted to come back but my body wouldn't allow me to," he said last year.
"I started to feel a little bit better last summer, so I told myself to give it a go and see how far I can take it."