Five-time Australian Open champion Serena Williams is in doubt for the year's opening Grand Slam after she was forced to pull out of the Brisbane International with an ankle injury on Wednesday.
The 13-time Grand Slam winner fell heavily and rolled her ankle during a baseline rally towards the end of her second round win over Serbia's Bojana Jovanovski.
Williams, who was serving for the match at the time, was able to continue following treatment and held on to beat Jovanovski 6-2, 6-4.
But the injury-prone player limped from the court and she was later forced to withdraw from her quarter-final against Daniela Hantuchova on medical advice.
"I'm disappointed to have to pull out of the Brisbane International," Williams said in a statement.
"I have been to hospital and it has been confirmed that I have a left ankle sprain that I probably shouldn't play on.
"I'm going to take a couple of days off - not too many - and see how I feel. I'm still hopeful of playing the Australian Open."
The 30-year-old Williams has been plagued by injuries since she won Wimbledon in 2010, beginning when she cut her foot in a German restaurant, keeping her out of the game for a year.
She came back in mid-2011 and had a series of good results before a toe complaint forced her to withdraw from a tournament in Canada.
She then took the last four months of 2011 off, conceding this week that she was tired at the time and needed a break.
Williams, who normally wears an ankle guard on her left leg, said she thought "not again" as she felt the ankle give way.
"I was like, no way, but as long as I was able to walk it was a little better (than in the past)," she said.
"I actually took (the ankle guard) off in the second set because it was painful because I was having another problem with my foot and it was hurting the other problem.
"I have a brace that I didn't put on today - I wasn't thinking."