American Danielle Collins stunned former champion Elina Svitolina 6-1 6-1 in the opening round of the Brisbane International on Monday as Germany's Angelique Kerber also bowed out.
World number five Svitolina entered the match as heavy favourite to progress but looked far from her best and failed to convert any of her four break points in the opening game.
Collins took her chances with aggressive hitting to grab an early break and went on to claim five consecutive games and clinch the opening set.
Fourth seed Svitolina struggled with her first serve throughout the match and Collins targeted her soft second serves to dominate the second set.
After losing the first game, Collins powered through six straight games to seal victory in 56 minutes.
It was a significant result for Collins, who has struggled for form since being diagnosed with rheumatoid arthritis in October.
The victory was Collins's third against a top-10 opponent and will lift her confidence ahead of the Australian Open, where she reached her maiden Grand Slam semi-final last year.
Earlier, home favourite Samantha Stosur upset former world number one Kerber 7-6(5) 7-6(4) in a tight clash between former Grand Slam champions.
Stosur, who has not progressed beyond the second round in nine Brisbane appearances, dazzled in the decisive tiebreaks with a combination of solid first serves and devastating forehands to clinch her first win over Kerber since 2015.
"This is sort of the way I want to be all the time on court," Stosur said.
"I put more pressure on myself than anyone could ever do, because I really know what I'm capable of.
"So sometimes I probably want it all too bad - and then that's when you kind of get hamstrung and don't perform as you want to. But today I certainly feel like I did stay relaxed and composed."
(Reporting by Hardik Vyas in Bengaluru, editing by Ed Osmond)
