Ostapenko begins Adelaide title defence with comeback win
The 17th-ranked Latvian, who won Roland Garros in 2017, will meet Madison Keys in the last 16 at the warm-up tournament leading into the Australian Open.
American Keys crushed Brazil's Beatriz Haddad Maia, ranked four places above her at 16, 6-2, 6-1.
Ostapenko struggled in a scratchy opening set marred by more than 20 unforced errors against Poland's Frech.
"After the first set I just fought for every point," the 27-year-old said. "I found my game and got better and better."
Keys, who married her coach Bjorn Fratangelo in November, broke four times in her rout of Haddad Maia.
"I didn't do much wrong -- and that doesn't happen very often," she said.
Greek lucky loser Maria Sakkari, who missed the latter part of 2024 with a shoulder injury, will next line up against top seed Jessica Pegula after defeating Czech Linda Noskova 6-3, 6-3.
"I'm coming back and have not had a lot of matches," the former number three said.
"Every match I play I feel better and my game is improving day by day."
Russian world number 13 Diana Shnaider also went through, beating Czech qualifier Katerina Siniakova.
In the men's draw American seventh seed Brandon Nakashima beat Germany's Yannick Hanfmann while Serb Miomir Kecmanovic upset Kazakh sixth seed Alexander Bublik.