Serena Williams

The 36-year-old had been planning to try and defend the title she claimed last year, for the seventh time in her superb career.

The 23-time grand slam champion joins the growing list of absentees for the year's opening major, which already includes five-time runner-up Andy Murray and former world number four Kei Nishikori, with doubts also over star duo Novak Djokovic and Rafael Nadal.

Serena has not played a competitive match since claiming a record-equalling slam at the Australian Open last year, having given birth to her first child in September.

Williams was around two months pregnant when she captured her 23rd grand slam singles title in Melbourne earlier this year and was hoping to defend her crown next January following the birth of her daughter in September.

"Serena is getting ready for 2018," the 36-year-old American's coach Patrick Mouratoglou told CNN. "She is practising and getting back in shape. I will be joining her soon in Florida.

"We will see how her body is reacting and how her tennis level is and we will take a decision regarding Australia."

Williams gave birth to a daughter, Alexis Olympia Ohanian, last month and Australian Open organisers have declared the seven-time champion will return to Melbourne Park in January.

The former world No 1 posted about her baby on Instagram and Twitter accounts and is heard saying in a video: "We had a lot of complications, but look what we've got."

The 35-year-old Williams said in late December she was engaged to Reddit co-founder Alexis Ohanian.

Williams wrote on Wednesday that their daughter was born on September 1 and weighed 6lb 14oz.

One posting also said about the newborn: "Grand Slam Titles: 1."

Williams won her 23rd Grand Slam singles championship at the Australian Open in January, while she was pregnant.

Williams and her fiancée Alexis Ohanian, the Reddit co-founder, welcomed their little girl on 1 September at a clinic in Florida.

The 23-time Grand Slam winner shared a video clip on her Instagram Stories, Facebook and website.

Williams announced the name of her baby daughter on her Instagram Stories profile. She also shared a montage of memorable moments from her pregnancy, including winning the Australian Open while pregnant and her first ultrasound.

The 35-year-old American, who is engaged to Reddit co-founder Alexis Ohanian, has not competed since winning the Australian Open in January but has posted several videos to social media showing her hitting balls during her pregnancy.

"Congratulations @SerenaWilliams for your baby girl. I am so happy for you and I feel your emotion," her coach Patrick Mouratoglou tweeted.

"Btw … I wish you a speedy recovery … we have a lot of work ahead of us."

McEnroe hit the headlines following an NPR interview over the weekend when he said, "If [Williams] played the men's circuit she’d be like 700 in the world." 

His comments came after the host, Lulu Garcia-Navarro, said Williams could be seen as the "best player in the world" instead of the "best female player."

Reaction to what McEnroe said has been strong, but when given the chance to apologise Tuesday on "CBS This Morning," the legendary seven-time major champion did not back down.

The professional athlete shared a lengthy message to Instagram on Monday in which she expressed her disappointment over Nastase's offensive remarks regarding Serena and fiancée Alex Ohanian's unborn baby. During a recent press conference held at the Fed Cup in Romania, Nastase was heard to have said of Williams' child, "Let's see what color it has. Chocolate with milk?"

The American, who is skipping the rest of the year after announcing last week that she was pregnant, took to social media to offer her first public thoughts on impending motherhood.

"My Dearest Baby, You gave me the strength I didn't know I had. You taught me the true meaning of serenity and peace," the 35-year-old who owns 23 grand slam titles, said on Instagram.

"I can't wait to meet you. I can't wait for you to join the players box next year," she continued.