Hingis, who underwent ankle surgery in May, struggled on her return from injury and announced in early October that she was taking an indefinite break in order to "free my mind".

The 22-year-old was expected to make her comeback at January's Sydney International tournament, at which she is a three-times former champion.

However, the event's director Craig Watson said he had been advised that the defending champion would be absent.

"The last indications that we got from Martina were that she was looking forward to being in Sydney," Watson told Australian newspaper The Age.

"But we've since been advised by the WTA that they don't think she'll be ready in time."

Should she miss the event, Hingis would be a major doubt for the Australian Open which begins on 13 January.

Retirement hint

Hingis, a five-times Grand Slam champion, recently admitted that she could not continue in the sport if she no longer felt able to win major titles.

She has not won a Grand Slam for nearly three years and in October she slipped out of the world top 10 for the first time in six years.

"I don't think just reaching quarter- and semi-finals would be good enough for me because I know I'm better than that," she said.

"I'm not going to put myself under pressure to return right now - I've been taking things slow and easy."