Top seed Iga Swiatek avoided an early Australian Open exit by overturning a double break deficit in the deciding set to beat Danielle Collins.
Swiatek trailed 4-1 but showed fight and composure as Collins wilted to win 6-4 3-6 6-4 in the second round.
"I was in the airport already," said the four-time Grand Slam champion.
American fifth seed Jessica Pegula became the highest-ranked casualty in the women's draw so far as she was beaten 6-4 6-2 by France's Clara Burel.
Burel, ranked 51st in the world, had never previously beaten a top 10 player but won comfortably in 70 minutes to set up a meeting with compatriot Oceane Dodin.
After Swiatek avoided an even greater shock by taking her third match point against Collins, the Pole dropped her racquet to the floor before hiding her head underneath a towel.
"I wanted to fight until the end," she said. "I knew she played perfectly but it would be hard for anybody to keep that level and I wanted to be ready when mistakes came from the other side.
"I wanted to push then, I did that and I'm proud of myself because it wasn't easy."
Third seed Elena Rybakina - who beat Swiatek in the fourth round last year - and two-time champion Victoria Azarenka are in action later on Thursday.
Swiatek overcomes another test in a tough draw
Swiatek, 22, has emerged as the WTA Tour's dominant player in the past two years, leading the world rankings for most of that time and adding three more majors to her tally.
Three of her four Grand Slam triumphs have come on the French Open clay, with the other on the US Open hard courts, but her attempt to add the Australian Open title this year has been complicated by a tough draw.
After beating 2020 champion Sofia Kenin in the first round, she faced another stern test against the uncompromising Collins, a finalist at Melbourne Park in 2022.
The American has dropped to 62nd in the world but brought her best game to unsettle Swiatek with her power.
Using her devastating backhand to great effect, Collins looked set to inflict Swiatek's earliest exit at a Grand Slam since the 2019 US Open.
However, Collins was unable to keep her nerve as the pressure of the finishing line approaching overwhelmed her.
Looking tight, she lost serve for 4-2, could not convert three break points the next game and lost her serve again as Swiatek turned the deficit into a 5-4 advantage.
Swiatek held two match points at 15-40 - missing one with a volley into the net and the other with a backhand from the baseline - but was given a third opportunity when Collins dragged a cross-court backhand way wide.
This time she took it. Scrambling to reach a dropshot after a long rally, she pushed a backhand down the line and celebrated extending her winning streak to 18 matches.
Afterwards, 30-year-old Collins revealed she intends for this season to be the final one of her career.
"I'm kind of at the end of my career and they [defeats] don't sting quite as much, to be honest," she said.
"I don't really know exactly when, but this will be my last season and I'm really looking forward to that."
Swiatek will face 19-year-old Linda Noskova in the third round, after the Czech beat American Mccartney Kessler 6-3 1-6 6-4.
BBC