Injury forces Williams and Sharapova out
World number one Serena Williams retired from her Hopman Cup match against Australian Jarmila Wolfe on Tuesday with a knee injury barely two weeks before launching her bid for a seventh Australian Open title.
Williams was trailing 7-5 2-1 when she decided not to continue after receiving medical treatment.
After pulling out of Monday's tie against Ukraine with inflammation in her knee, the 34-year-old American hobbled through the match before deciding she could not go on.
Playing for Australia Gold, Wolfe took the first set as Williams struggled to move to her left.
"I have some knee inflammation that's going away very slowly. It's going, it just needs a little more time," Serena told reporters, refusing to rule out playing in the Americans' Hopman Cup match against Czech Republic on Thursday.
"I'm not making any decisions. I'm just taking it one day at a time," the 21-times grand slam singles champion said.
Meanwhile, Maria Sharapova and Simona Halep have withdrawn from the Brisbane International to make it a worrying start to the season for tennis' leading women.
Sharapova was the defending champion in Brisbane but pulled out prior to her opening match because of an arm problem while top seed Halep is still struggling with a niggling Achilles injury.
The Romanian, who is hoping to play in Sydney next week, said: "I had problems with my Achilles in August and September, and then I didn't feel it any more, but now I've already had a couple of weeks again with the pain.
"I thought I would be ready to play here."