Doubt lingers over the fitness of tournament favourite Serena Williams after she switched her training schedule on Wednesday to practise on indoor courts away from prying eyes.

Williams, the third seed, rolled her ankle during her first-round win over Romanian Edina Gallovits-Hall.

She was forced to take an injury time-out and had the right ankle heavily taped but went on to complete a 6-0 6-0 win.

Williams was upbeat about the issue afterwards but said she would know whether it was a serious problem within 24 hours.

The American’s second-round appearance remains under a cloud after she told tournament officials late on Wednesday that she wanted to train indoors in private, away from media and the public.

She had originally planned an outdoor session on court 17.

Hundreds of spectators were 10-deep around the outside court until it became clear that the five-time Australian champion would not be making an appearance. Instead, she was hitting balls indoors where she had her ankle heavily strapped.

Television cameras captured the star moving gingerly as she talked with her coach.

Williams said after her match she might have scans depending on how the injury develops.

“That’s definitely optional. I’m going to play it by ear,” Williams said on Tuesday.

“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.