Kim Clijsters is a doubt for the Australian Open after limping out of the Hopman Cup with an injury.
Clijsters was treated in the third set
The Belgian star had to retire in the third set against Australia's Alicia Molik with an ankle injury.
The former world number one began clutching her left ankle in the third set and received court-side treatment.
Tournament director Paul McNamee would only say doctors are "cautiously optimistic" Clijsters will be fit for the start of the Open on 19 January.
"Obviously she was disappointed when she left here but it is what it is," he added.
"The MRI scan shows bruising on the bone at the back of the ankle joint.
"She needs a combination of rest and treatment. She is keen to do the treatment and get on with it."
Clijsters had been in mixed form in the match, winning the first set 6-3 but then failing to convert four match points in the second.
She was leading 3-2 in the deciding set when she was forced to retire.
The withdrawal disappointed more than 8,500 fans, who had packed into the stadium to watch a mixed doubles match involving Clijsters and her fiance Lleyton Hewitt.
"I saw Kim afterwards and she was pretty distressed," said Hewitt. "I guess she will just be hoping she can make the Australian Open later this month."
Clijsters' injury, combined with victory for Hewitt over Xavier Malisse, booked Australia's place in the next round.
Hewitt bounced back after losing the first set to win 3-6 6-1 6-2, while Australia took the tie 3-0 with a walkover in the mixed doubles.
Hewitt suffered a scare when he twisted his ankle in the third set, but said it was not serious.
"It was lucky I was wearing my ankle braces otherwise I'd probably be out of the whole Australian summer," he said.
"I felt it twist pretty badly and the ankle braces controlled me before I fell over, which was a good thing. Hopefully it will be right."
The United States secured a comfortable passage into the next round with a 3-0 win over France.
Lindsay Davenport beat Amelie Mauresmo 6-4 6-4 and James Blake saw off Fabrice Santoro 6-3 6-4, before Davenport and Blake beat Mauresmo and Santoro 7-5 6-1.