Continuing her march towards a record 24th Grand Slam title, Serena Williams is adamant she has another level to go to after vanquishing world number one Simona Halep in a high-quality fourth round contest at the Australian Open.
Williams blazed through the first set in just 20 minutes but had to fight tooth and nail on Rod Laver Arena to put away the valiant Halep, prevailing 6-1 4-6 6-4 to book a quarter-final clash with Czech seventh seed Karolina Pliskova.
The victory, just 16 months after giving birth to daughter Olympia, secures an astonishing 50th grand slam quarter-final appearance for the 37-year-old American.
Having dropped a miserly nine games on her path to the fourth round, Williams raced through the opening set and looked primed to make short work of her Romanian opponent.
Halep fought back admirably but her supreme defensive game wasn't enough to stop Williams, who stared down three break points while trailing 3-2 in the deciding set.
The seven-time Australian Open champion nonetheless played down her form heading into the final eight.
"I think overall I'm hanging in there ... I'm solid," Williams said.
"I can definitely go to a new level. I have to if I want to stay in the tournament."
Playing in just her eighth tournament since becoming a mother, Williams conceded she was still finding her way back to her best.
Williams looked briefly vulnerable during the second set when Halep worked her around the court, roaring and pumping her fists after fighting back to even the ledger.
"That's why she's number one," Williams said.
"She literally lifted her game to a new level. I didn't. I kind of stayed at the same level, and I should have looked at my game as well.
"But it's a part of this journey on my way back. It's 10 months so I can't be too upset at myself. I felt like I did have an opportunity to win that in straight sets but then I'm playing the number one player in the world.
"I'm still learning which is, at my age and my point in my career, I think admirable and exciting that I still have things I can learn from."
The defeat leaves Halep at risk of losing her number one ranking with Japan's Naomi Osaka among four rivals who could ascend the summit depending on their remaining results.
Halep was philosophical about the prospect of losing her top ranking.
"There is another chance after this tournament to go back to number one if I lose it," she said.
"The main goal is just to play as good as I can every match, to win every match I play, so the ranking doesn't really matter."