World number one Simona Halep's first match in more than three months ended in defeat by Australia's Ashleigh Barty at the Sydney International.
The Romanian, who cut short her 2018 season with a back injury, was beaten 6-4 6-4 by the world number 15 in the second round after a first-round bye.
French Open champion Halep, 27, who was runner-up at last year's Australian Open, has started 2019 without a coach.
Barty fired 26 winners on her way to her first victory over a world number one. She will face Belgian 10th seed Elise Mertens in the next round.
Despite the defeat, Halep was pleased that she had played "a good level of tennis" and was not troubled by her back.
"No pain at all, that is a great sign," she said. "The tennis is good, I just have to believe in myself more."
Former US Open champion and fourth seed Sloane Stephens followed Halep out of the tournament after a 3-6 7-6 (7-4) 6-0 defeat by Kazakh world number 44 Yulia Putintseva.