Defending Australian Open champion Naomi Osaka squandered two match points as she suffered a shock third-round exit at the hands of unseeded Amanda Anisimova on Friday.
Osaka eased past Camila Osorio and Madison Brengle in straight sets in the first two rounds but fell to a 4-6 6-3 7-6 (10-5) loss to Anisimova in a thrilling contest.
Anisimova, who won her second career title at Melbourne Summer Set 2 earlier this month and is now 8-0 for the season, will face top seed Ash Barty in the last 16.
Two-time Australian Open winner Osaka had won 24 of her past 25 matches in Melbourne and made a fast start against Anisimova by breaking her opponent in the first game.
She held serve to ease ahead on Margaret Court Arena, but Anisimova battled back well in the second set.
The world number 60 broke Osaka in the fourth game with a sublime disguised backhand drop and served out the set in a relatively straightforward manner.
Anisimova continued to dig in and saved two match points in the 10th game to help pave the way for a decisive first-to-10 tie-break.
Osaka had no response to her opponent's accurate hitting as she fell 3-0 behind and, while she did bring it back to 3-2 and 5-4, Anisimova took five of the next six points to advance in a big upset.
DATA SLAM: Osaka outgunned by Anisimova
Osaka had dropped just one set in her first five matches this season and looked in the mood as she raced ahead against Anisimova.
But the American youngster fired in 11 aces and hit more than twice the number of winners that Osaka managed (46 to 21) to pull off the victory.
With this defeat, Osaka has now failed to defend any of her four major crowns, with Victoria Azarenka the last women's player to do so at Melbourne Park in 2012 and 2013.
Anisimova – 46/44
Osaka – 21/45
Anisimova – 11/8
Osaka – 5/3
BREAK POINTS WON
Anisimova – 1/8
Osaka – 1/10