Serena Williams has withdrawn from the season-opening Grand Slam in Melbourne and will not defend the title she won in 2017.
The 36-year-old was hoping to return to the court after the birth of her daughter Alexis Olympia in September.
The 23-time Grand Slam winner defeated her sister Venus in last year's Australian Open 6-4 6-4 and had a second consecutive title in her sights when the tournament started later this month.
She had won the tournament seven times, with her first victory coming in 2003.
But on Friday morning the former world number one said she was withdrawing from the tournament, which starts on January 15.
She said: "After competing in Abu Dhabi I realised that although I am super close, I'm not where I personally want to be.
"My coach and team always said 'only go to tournaments when you are prepared to go all the way'.
"I can compete - but I don't want to just compete, I want to do far better than that and to do so, I will need a little more time."
Ms Williams returned to the court on December 30 at the World Tennis Championship in Abu Dhabi, where she lost an exhibition match against Jelena Ostapenko 6-2 3-6 10-5.
Australian Open Tournament Director Craig Tiley said: "The true champion Serena is has been demonstrated in the herculean efforts she has made over the past few months in her desire to play the Australian Open.
"Serena transcends the sport in the way she approaches all aspects of her life and consistently gives her all in everything she does.
"It was never going to be good enough for her to just compete, she wants to give herself the best chance to win.
"I've been in constant contact with Serena and her team and know this is why she has pushed it and pushed it until the eleventh hour to make her final decision.
"We all wish her the very best and I look forward to seeing her back on court this year, and can't wait to welcome her back to the Australian Open in 2019."