With three singles majors more than 15-times grand slam winner Williams, Martina Navratilova this week unsurprisingly stopped short of agreeing with McEnroe's lofty rating.
But Navratilova has no doubts Williams is high on the list with the likes of Chris Evert, who also has 18 slams, Graf (22) and Australia's all-time leader Margaret Smith Court (24).
"She is not done yet - definitely in top five, top three, I don't know," Navratilova said.
"I never rate the greatest because the different generations, different parameters, so you can't really judge that.
"It is just nice to be in that group and she is definitely in that group.
"She is playing amazing tennis now. She seems to be very happy. She's happy off the court and happy on the court and I think she realised that her time was running out and re-dedicated herself.
"That first-round loss at the French Open last year was the best thing that happened to her and now she's seeing how much fun she's having winning everything."
With her own Open hopes looking forlorn after another horror build-up, Samantha Stosur - who has enjoyed more success than most against Williams - declared the American a raging favourite.
"I think you would be a pretty brave person to go against her straight off the bat," said Stosur, describing Williams' French Open flop as a merely a "bit of a mishap".
"She's the in-form player and has carried that for a really long period of time now."
In a statistical anomaly, world No.1 and defending champion Victoria Azarenka and 2012 Open runner-up Maria Sharapova remain ranked above Williams.
Apart from owning dreadful head-to-head records against Williams, Azarenka (toe) and Sharapova (shoulder) both arrived in Melbourne after pulling out of the season-opening Brisbane International and skipping the Sydney lead-up event.
Polish fourth seed Agnieszka Radwanska, German fifth seed Angelique Kerber and Chinese sixth seed Li Na loom as possible challengers after making impressive starts to 2013.
But the Williams juggernaut will take a power of stopping.
"I am playing some of my best tennis," Williams said.
"I feel like I want to do better and play better still."
-AAP