Seven-time champion Serena Williams has withdrawn from the Australian Open because of concerns about her fitness four months after giving birth to her first child.
Williams gave birth to a daughter in September and was intending to return to defend her crown in Melbourne.
But the 36-year-old, who has won 23 major singles titles, said her fitness was not at the level she had hoped after appearing in an exhibition match last week in Abu Dhabi, where she lost to French Open champion Jelena Ostapenko.
"After competing in Abu Dhabi, I realised that although I am super close, I'm not where I personally want to be," Williams said in a statement.
"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.
"With that being said, and even though I am disappointed about it, I've decided not to compete in the Australian Open this year.
"The memory of last year's Open is one that I will carry with me, and Olympia and I look forward to coming back again.
"I appreciate the support and understanding of my fans and everyone at the Australian Open."
Williams beat older sister Venus in last year's final before revealing she played the tournament despite being more than two months' pregnant.
She had been desperate to defend her crown and equal Australian great Margaret Court's all-time record of 24 major singles titles.
Williams' scratching comes less than 24 hours after Andy Murray withdrew from the men's event with a chronic hip injury.
Several other big names, including top-ranked Rafael Nadal, six-time champion Novak Djokovic and 2014 winner Stan Wawrinka, also remain under fitness clouds 10 days out from the year's first major at Melbourne Park.
Williams' withdrawal leaves women's field wide open
Williams' absence, while disappointing for fans and officials, raises the prospect of another first-time major winner making their mark in Melbourne.
Latvian trailblazer Ostapenko in Paris and American Sloane Stephens at the US Open both won their first majors last year while Williams was sidelined.
Five different players also shared the top ranking in 2017 after Williams was dethroned during her hiatus.
Romanian Simona Halep, the current world number one, and Dane Caroline Wozniacki are jostling for women's top seed status in Melbourne.
Australian Open tournament director Craig Tiley expressed his admiration for Williams' efforts to return to the game she loves.
"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," Tiley said.
"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."