MELBOURNE - Defending champion Serena Williams announced that she would withdraw from the Australian Open on Friday, adding that she was not fully prepared to keep playing after giving birth four months ago.

"My coach and team always said 'only go to tournaments when you are prepared to go all the way,'" Williams said. "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," she added.

Last January, Serena won the Australian Open for an Open-Era record seventh time, defeating her sister, Venus, in the final. This was her 23rd Grand Slam singles title, surpassing Steffi Graf's Open Era record of 22.

Later in April, she announced that she would miss the reminder of the season due to being 20-week pregnant, which means that she was two months pregnant when she won the Australian Open.

"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," said the 36-year-old new mom.

The 23-time Grand Slam winner played her first match since giving birth at the World Tennis Championship in Abu Dhabi last year on December 30, losing her exhibition match to reigning French Open Champion Jelena Ostapenko.

"After competing in Abu Dhabi, I realized that although I am super close, I'm not where I personally want to be. With that being said, and even though I am disappointed about it, I've decided not to compete in the Australian Open this year," she said.

The absence of the American enlarges the growing list of no-shows at this year's Australian Open. Five-time finalist Andy Murray announced his withdrawal due to a hip injury on Thursday, while Japanese player Kei Nishikori pulled out of the year's first Grand Slam due to a torn tendon in his right wrist. World No. 8 Jack Sock also joined the list when he injured his hip at the Hopman Cup on Tuesday.

Simultaneously, 16-time Grand Slam winner Rafael Nadal and former world No. 1 Novak Djokovic's attendance remains in doubt due to uncertain injuries.