The Belarusian player hit 32 winners to beat Mertens, with whom she won the doubles title in Melbourne in 2021.
“It’s really tough to play against her, she’s a great player and is always fighting,” Sabalenka said.
Sabalenka will now play Belinda Bencic after the Swiss player reached the fourth round for the first time since 2016, beating Camila Giorgi 6-2, 7-5 on Rod Laver Arena. The 12th-seeded Bencic was broken when serving for the match at 5-4 but she broke serve in the following game and then served out to love for victory.
“I’m really happy I got a second chance to serve it out,” she said. “But overall I think it was a great match. [At 6-5] I just tried to relax. My coach [Dmitri Tursunov] was showing me relax [signals].”
Bencic won the warm-up event in Adelaide before the Australian Open and has won eight matches in a row.
“My serve, it can be a really good weapon,” she said. “I’m not doing as much running from the baseline.”
Information from The Associated Press was used in this report.