Reigning Australian Open champion Serena Williams and two-time winner Victoria Azarenka will miss the first major of the year, leaving the women's draw wide open when the Grand Slam season kicks off Down Under later this month.

Williams, a seven-time victor in Melbourne, withdrew on Thursday while Azarenka, who lifted back-to-back trophies in 2012 and 2013, officially pulled out on Sunday, according to tournament organizers.

Williams, who was eight weeks pregnant with her first child, Alexis Olympia, when she won the title 12 months ago, said that while she is encouraged by the early stages of her comeback, she needs more time to return to the form that has won her 23 Grand Slam titles.

"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," said Williams in a statement. "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."

Australian Open tournament director Craig Tiley was full of kind words for both champions, praising Williams' "herculean efforts" to return to competition just four months after giving birth to her first child and saying Azarenka was looking forward to returning to Melbourne in 2019.

Williams competed in an exhibition match last week at the Mubadala World Tennis Championship in Abu Dhabi, where she narrowly lost to reigning Roland Garros champion Jelena Ostapenko. She has not set a date for her first official tournament back, but has indicated that she will return to the tour as soon as she feels she is producing her best tennis. 

The six-time US Open champion has indicated her main goal is to tie and ultimately surpass the record 24 Grand Slam singles titles of Australian Margaret Court.

"Knowing that I have won 23 Grand Slam titles and several other titles, I don't think I have anything more left to prove," she said. "But I am not done yet."

Azarenka, meanwhile, has played in just one Grand Slam tournament since a first-round exit in the 2016 French Open. After giving birth to son Leo in December 2016, the Belarusian reached the fourth round of Wimbledon but missed the 2017 US Open and the 2017 Fed Cup final with an ongoing personal matter.

The Australian Open begins Monday, January 15.