Serving only so-so, failing to convert all but one of her break chances and missing groundstrokes with alarming regularity, Williams stunningly exited in the third round at Melbourne Park, beaten 6-4, 6-7 (2), 7-5 by 27th-seeded Wang Qiang of China on Friday.
The last set went with serve until the 12th game when Wang secured an all-important break to seal the match and her passage to the next round.
Looking at her opponent, Serena Williams has continued her dominant performance as she won her previous match in straight sets.
"I definitely do believe (I can equal it) or I wouldn't be on tour", said Williams, a seven-time victor at Melbourne Park.
"After last time, I've done a lot of hard work on the court and off the court. I wanted to be number one in the world", said Wozniacki, the Melbourne victor in 2018. I just care about trying to keep going, keep enjoying my tennis career.
The world number two Serb, who struggled through a four-set grind against unseeded German Jan-Lennard Struff in his opener, was back to his imperious best as he fired down 16 aces and racked up 31 winners at a gusty Rod Laver Arena. "Each one is definitely an opportunity for me to go out there and win", Williams told reporters.
He gave her a kiss on the cheek by way of an apology, then continued on his way to a 6-3 7-6 (7-4) 6-1 victory.
But Slovenian Zidansek found her rhythm in the second set and began to make much more of a fight of it. I'm starting to feel very, very comfortable and compact on the court.
It is a big ask for the 33-year-old Nadal, who has only won the title once before in Australia, against a tearful Federer in 2009, with four runner-up finishes to his name.
Williams's shrieking hit ear-splitting levels but Wang, unintimidated, calmly served out the set. If I reach 21, better.
"I don't know. I seemed to do well the last two slams of the year".
Djokovic beat Spain's Rafael Nadal in the final of last year to unleash a year during which he also crowned his fifth title in the Wimbledon English championship, in addition to five tournaments during which he raised his tally to 77 titles in his professional career.