The world number one tamed French wildcard Tiantsoa Rakotomanga Rajaonah 6-4, 6-1 as she bids to claim a third Melbourne crown in four years.
The Belarusian faces Anastasia Pavlyuchenkova of Russia or Chinese qualifier Bai Zhuoxuan next.
Sabalenka, who lost last year's final, said that the first match of a Grand Slam was always tricky, especially against an opponent she had never played before.
Sabalenka has been trying to expand her game, including going to the net more often, and said the variation made her a better player.
"I was definitely working on serve and volley. I'm super happy to be able to do it in the match," she said.
"You know, on the practice (court) is one thing, but to be able to be kind of fearless and go there at the match is something else.
"I'm really proud I'm able to show this tennis."
Sabalenka makes sloppy start
The 27-year-old, fresh from winning the season-opening Brisbane International, made a sloppy start in losing the first three points of the match when serving.
In front of the watching Roger Federer, Sabalenka clawed back to deuce but then misfired again to fall a game down in front of a shocked Rod Laver Arena.
Sabalenka, who is the favourite to add a fifth Grand Slam title to her career haul, pulled back level, but if she expected the spirited world number 118 to melt away, it did not immediately happen.
The reigning US Open champion offered an apology to her underdog opponent at 4-4 when she unleashed a forehand into the 20-year-old's head.
Sabalenka's error count was rising, but she finally broke Rakotomanga Rajaonah's resistance to claim the first set, then eased home in the second.
"I was super-happy I found my level," said Sabalenka, who was guilty of 13 unforced errors in the first set.
"It is always tricky playing someone young, someone you don't know, and a lefty (left-hander)."
Sabalenka said she had felt pressure with Federer and Rod Laver watching court-side.
"I hope you guys enjoyed watching me play, I hope you enjoyed it even a little bit," she said in her on-court interview.
"Can I take a picture with you, please?"
Sabalenka was stunned in the Melbourne final by Madison Keys of the United States a year ago.