Former Australian Open semi-finalist Sloane Stephens will miss the 2017 edition of the tournament due to an ongoing foot problem, organisers announced.
Stephens reached the last four in Melbourne in 2013 - her best grand slam performance - but was unable to make it to the final as she was beaten in straight sets by eventual champion Victoria Azarenka.
It will be the second successive grand slam that the 23-year-old is unfit to play after missing the US Open last September - Stephens's last appearance coming at the Rio Olympics when she was beaten by Eugenie Bouchard.
While Stephens is out of the opening grand slam of 2017 there are injury concerns for two of the world's top 20.
Hours after winning the seventh WTA title of her career in Brisbane, Karolina Pliskova withdrew from the final warm-up tournament ahead of the Australian Open through injury.
Pliskova - who will rise to world number five on Monday - battered Alize Cornet on Saturday but it may have come at a cost as she was forced to withdraw from the Sydney International with a thigh problem.
World number 14 Elina Svitolina is another doubt after she also pulled out of competing in Sydney due to a viral illness.
Svitolina had been due to face Monica Puig on Sunday in the first round of the WTA Premier event, but was forced to withdraw, saying: "I've had constant headaches for the last two days and unfortunately in the hot weather it got even worse when I was warming up and my body wasn't ready to compete."