The Belarusian top seed ripped through the first set in 29 minutes.

Sabalenka let her momentum drift at the start of the second set as two unforced errors and a double fault allowed Kasatkina to break in the opening game. The Australian then held to love as Sabalenka continued to miss.

The Belarusian took the next three games but faced greater resistance from Kasatkina.

At 6-5, with Kasatkina serving, Sabalenka reasserted her control. She raced to three match points and converted the second to win in one hour and 17 minutes, and become the ninth woman to win 100 matches while ranked No.1.

"I was happy in the tough moments I never gave up," she said.

She said that while she would have been happy to cruise through the first week of a major with easy wins, Kasatkina's resistence could be beneficial.

"Good to be tested, like to feel the fight like activate this fight mode, to feel the pressure a little bit. I feel like it's a good preparation for the second week," she explained.

In the round of 16, Sabalenka will face fellow four-time major winner Naomi Osaka.

Osaka won their only meeting at a major, at the same stage in the 2018 US Open, a tournament the Japanese star went on to win for the first of her four majors.

Since then they had not met again until two encounters this season -- both won by Sabalenka at Indian Wells and Madrid.

"It's great, great to see her back on her level. Maybe not at her best level, but she's back, she's fighting, she's building her level," said Sabalenka.

"I'm just ready for the fight. I'm ready to go out there to fight for that match, for that win. Ready to do anything it takes to get the win."