Boulter beat American Coco Gauff in the second round of the Australian Open warm-up and another upset looked on the cards when she matched her Japanese opponent’s power-hitting to take the opening set on Margaret Court Arena.
The second seeded Osaka, who won the Australian Open in 2019, raced to a 3-0 lead in the second set but the 24-year-old Briton would not fold and had break points to put the stanza back on serve.
Osaka upped her return game considerably to wrest back the momentum and level up the match before running away with the third set and sealing a last eight tie against another Briton in Johanna Konta or Irina-Camelia Begu.
“After I lost the first set I thought I just want to fight as hard as I can,” said Osaka, who is playing her first WTA tournament since securing her second title at Flushing Meadows in September.
“She was playing really amazing and I thought ‘if I lose okay, but I have to give it my best shot’. I like to play long matches ... and I played a short match yesterday so maybe I made myself play a long match today.”
Top seed and world number two Simona Halep is in action in her third round match against German Laura Siegemund on the same court later on Wednesday. Reporting by Nick Mulvenney in Sydney; Editing by Michael Perry