The seven-times champion scrapped her way to a 6-4 2-6 6-4 win over world No.7 Belarusian Aryna Sabalenka on Sunday.

The 39-year-old American, 17 years older than her rival, was up 4-1 in the third set but had withstand a fierce fightback from Sabalenka to advance after two hours and nine minutes.

With loud grunts from the power pair making up for the absence of fans on Rod Laver Arena, they delivered a high-quality showing in their first ever meeting.

Williams said her early career clashes with former world No.1 and fellow American Lindsay Davenport had prepared her for anything hard-hitting Sabalenka threw at her.

"I don't know anyone that hit harder than Lindsay - she was the most power player I think I've ever played," Williams said.

"It was definitely a lot of power ... but I'm used to it.

"I was OK with it really; If she wants to play power, let's go."

Sabalenka had only reached the final 16 once at a grand slam tournament - the US Open in 2018 - but was one of the form players heading into the year's first major.

The 22-year-old had won 18 of her past 19 matches, including the WTA 500 title in Abu Dhabi last month which was her third straight title.

After edging her rival in the first set, Williams was broken early and trailed 3-0 in the second and, after falling heavily during the set, the American looked like she could be on the ropes.

But in her usual fashion, Williams came out firing in the third.

Sabalenka rallied back from 4-1 down to level at 4-4 before Williams broke her again to seal the win.

"I am happy to get through that one," said Williams, who served eight double faults along with nine aces.

"It wasn't easy - I knew it wasn't going to be easy - and she was teeing off on every shot."

Williams said she didn't let herself get disheartened by the second-set scoreline.

"Even the games I lost were super close and I was like 'Serena, you've got this just keep going'," the 10th seed said.

"I lost the second but the games were so close it could have gone either way so I just tried to stay in there."

Williams will next take on two-time major winner Simon Halep, the second seed, who ground out a tough 3-6 6-1 6-4 win over reigning French Open champion Iga Swiatek.

Williams has won nine of her 11 career meetings with Halep.

Australian Associated Press