Sloane Stephens followed Simona Halep out of the Sydney International after Yulia Putintseva served up a final-set bagel.
World number one Halep was beaten 6-4 6-4 by Ashleigh Barty in her first competitive match since recovering from a back injury.
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Stephens also failed to progress to the quarter-finals later in the day, the unseeded Putintseva beating the 2017 US Open champion 3-6 7-6 (7-4) 6-0.
Fourth seed Stephens was beaten by Johanna Konta in the first round of the Brisbane International last week, and the American had to recover from a first-set bagel to defeat qualifier Ekaterina Alexandrova in the opening round in Sydney.
The world number five was blown away by Kazakh Putintseva, who is ranked 44th in the world, in the deciding set having been only one game away from a straight-sets victory.
What a win for @PutintsevaYulia!
— WTA (@WTA) January 9, 2019
Completes the stunning comeback victory over Stephens 3-6, 7-6(4), 6-0 at the @SydneyTennis! pic.twitter.com/TVxcEGDdzV
Top seed Halep said she will head into the Australian Open, the first grand slam of the year, with no expectations after going down in straight sets to Barty.
Petra Kvitova will face defending champion Angelique Kerber in the last eight, having sent Hsieh Su-wei packing courtesy of a 7-6 (7-2) 6-2 success.
Elise Mertens, Aliaksandra Sasnovich and Timea Bacsinszky were also second-round winners.
Sofia Kenin followed up her victory over top seed Caroline Garcia by beating Ons Jabeur 7-6 (8-6) 6-3 to reach the quarter-finals of the Hobart International.
Belinda Bencic, conqueror of second seed Mihaela Buzarnescu in round one, dumped out Zoe Hives 6-2 6-4.
Anna Karolina Schmiedlova saw off eighth seed Alison Van Uytvanck, while Alize Cornet, Kirsten Flipkens, Greet Minnen, Irina-Camelia Begu and Dayana Yastremska all came through their second-round matches.
