The 23-year-old Osaka became the third player to win each of their first two major meetings against Williams. Williams’ sister, Venus, won her first three major meetings at the 1998 Australian Open, 2000 Wimbledon and 2001 US Open. The other is Jennifer Capriati at the 2001 French Open and 2001 Wimbledon, according to ESPN Stats & Information research.
Williams was trying to reach her ninth Australian Open final in a bid to win a record-tying 24th Grand Slam.
Williams grabbed the first two games before Osaka took control with a five-game run. Osaka broke Williams to begin the second set and was on her way.
Williams’ forehand was off the mark all match and she finished with totals of 24 unforced errors and just 12 winners.
The Associated Press contributed to this report.