Defending champion Elina Svitolina started her season with a defeat to Aliaksandra Sasnovich and Petra Kvitova was also knocked out of the Brisbane International on Wednesday.
Svitolina ended 2018 by winning the biggest title of her career at the WTA Finals, but Sasnovich beat the top seed 6-4 0-6 6-3 in the world number four's first match of the new year.
Sasnovich responded with great character following a second-set bagel and struck 32 winners as she marched into the quarter-finals.
Svitolina defeated Sasnovich in the final of this tournament last year, so it was sweet revenge for the world number 30 from Belarus.
Kvitova bowed out earlier in the day with a 7-5 7-6 (7-1) loss to the unseeded Anett Kontaveit.
Kontaveit reached a career-high 20th in the rankings last year and could make further strides in 2019 on the evidence of a composed display against the fourth seed.
Kiki Bertens was another seed who was bundled out, with Donna Vekic consigning the world number nine to a 7-6 (7-5) 1-6 7-5 loss.
Johanna Konta was brought back down to earth a day after shocking Sloane Stephens, Ajla Tomljanovic seeing off the Brit 6-2 7-6 (7-2) in her homeland.
Karolina Pliskova, the fifth seed, came out on top in an all-Czech battle with Marie Bouzkova.