World number one Ashleigh Barty suffered a shock loss to qualifier Jennifer Brady in her first match of the new season on home soil at the Brisbane International.
Barty will carry the hopes of her nation at the Australian Open later this month but was unable to make it past the second round in Brisbane, with American Brady winning 6-4 7-6 (4).
The victory made it a notable double for Brady, ranked 53, after her first-round win against Maria Sharapova.
Barty said on wtatennis.com: "Jen was in control of the match. She was in the centre of the court being the aggressor and I think you have to give credit where credit's due."
Australian Open champion Naomi Osaka lost the opening set to Sofia Kenin but finished strongly in a 6-7 (3) 6-3 6-1 victory.
Defending champion Karolina Pliskova began her campaign with a hard-fought 6-4 6-7 (5) 6-1 victory over Australian Ajla Tomljanovic while Petra Kvitova eased past Liudmila Samsonova 6-3 6-2.
Serena Williams lost the opening set of her second-round clash with Christina McHale at the ASB Classic in Auckland but recovered to win 3-6 6-2 6-3.
Williams will make her latest bid for a record-equalling 24th grand slam singles title in Melbourne, and she said in her one-court interview: "It was a good match for me, because I really need to play matches like that to get to my goals.
"I got myself in a hole and I was able to get out of it, so that was satisfying."
There will be no first meeting between Williams and 15-year-old Coco Gauff in the quarter-finals, though, after the teenager was beaten 5-7 6-2 6-3 by Germany's Laura Siegemund.
Williams' doubles partner Caroline Wozniacki also dug herself out of a hole in a 6-1 4-6 6-4 victory over American Lauren Davis and next faces defending champion Julia Goerges, who defeated Jil Teichmann 6-3 6-2.
Garbine Muguruza battled into the semi-finals at the Shenzhen Open with a 6-4 2-6 6-4 victory over Zarina Diyas.
There she will face fifth seed Ekaterina Alexandrova, who defeated home hope Wang Qiang 3-6 6-4 6-3, while Elena Rybakina upset third seed Elise Mertens, winning 6-4 4-6 6-2.