Top seed and defending champion Angelique Kerber suffered a fourth-round exit from the Australian Open to world number 35 Coco Vandeweghe.
German Kerber, 29, claimed Grand Slam titles at Melbourne and the US Open last year, but lost 6-2 6-3.
American Vandeweghe, 25, faces a last-eight meeting with Spain's French Open champion Garbine Muguruza, who beat Romanian Sorana Cirstea 6-2 6-3.
Venus Williams also progressed, beating Germany's Mona Barthel.
The 36-year-old seven-time Grand Slam champion - the oldest player in the women's singles - won 6-3 7-5 and will take on Anastasia Pavlyuchenkova, who beat fellow Russian Svetlana Kuznetsova 6-3 6-3.
Vandeweghe's victory was her first against a world number one and takes her to a Grand Slam quarter-final for the second time in her career, after reaching the last eight at Wimbledon in 2015.
It meant the top seed from the women's and men's draws both left the competition on day seven, after Andy Murray's shock defeat by Germany's world number 50 Mischa Zverev.
Vandeweghe had Kerber on the defensive for much of their 68-minute contest, closing out the first set with ease after breaking in the sixth game.
And even though Kerber was able to break immediately in the second set, Vandeweghe won four successive games from 3-2 down for the biggest win of her career.
"It was really special," she said. "Beating the world number one on any stage and any place is great. I'll take this one.
"I wasn't feeling confident - I guess I faked it. I was nervous, but I had a game plan to execute and I knew that as along as I kept picking my spots, I had a chance."
Venus relishing another opportunity
It is 19 years since Venus Williams reached her first Australian Open quarter-final, and 14 years since her one and only final, when she lost to her sister Serena.
In recent years she has had to cope with the autoimmune disorder Sjogren's syndrome - but has managed to reach at least the last 16 at six of the last seven Grand Slams.
Barthel, ranked 118th in the world, had played Williams twice before but missed four months of tennis last year because of chronic fatigue illness.
"I know what it's like to be down on your luck, but she knows how to play tennis and she's experienced," Williams said.
"We'd played a couple of tough matches before so today I expected to have some competition."
Williams, who took one hour and 36 minutes to beat Barthel, has yet to drop a set at this year's Open.
"I've been blessed enough to do something that I love and I think this was my calling because I grew so tall that I can cover the court and hit it hard," she added.