Brisbane - American Danielle Collins thrashed fourth seed and former champion Elina Svitolina in the first round of the Brisbane International on Monday as Germany's Angelique Kerber also crashed out.
Svitolina, the world No 5, went into the match as firm favourite but was never in the hunt against Collins, going down meekly 6-1, 6-1 in just 56 minutes.
Collins dominated every facet of the match against a disappointing Svitolina, who won this tournament in 2018.
The American served well and returned even better, winning 62 percent of the points off Svitolina's first serve and 61 percent off her second.
She joins two other US players in the second round following wins by Sophia Kenin and Alison Riske.
Kenin beat Latvia's Anastasija Sevastova 7-6 (7/1), 6-4 while Riske downed Czech player Karolina Muchova 6-4, 6-2.
In a battle of former Grand Slam champions, Australia's Samantha Stosur upset former German world number one Kerber in two tough sets.
Stosur, 35, has slipped to 98 in the world rankings and has never played well in front of her home fans in Brisbane.
But with new coach - and former doubles partner - Rennae Stubbs at her side, Stosur played a strong tactical game to overcome Kerber.
"She knows all the players, her tactical awareness and everything about the players is excellent," Stosur said.
"That was something last year when I was going through without a coach, I found hard to do that day after day.
"Even though we know a lot of players and you know what to do, having that other set of eyes doing that is obviously very helpful."
In other matches, Czech Barbora Strycova upset seventh seeded Briton Johanna Konta 6-2, 3-6, 6-3, and Ajla Tomljanovic beat fellow Australian Priscilla Hon 6-2, 4-6, 6-4.
Results from the first day of the WTA Brisbane International tennis
tournament at the Queensland Tennis Centre on Monday (X denotes seed):
Barbora Strycova (CZE) bt Johanna Konta (GBR X7) 6-2, 3-6, 6-3
Sofia Kenin (USA) bt Anastasija Sevastova (LAT) 7-6 (7/1), 6-4
Alison Riske (USA) bt Karolina Muchova (CZE) 6-4, 6-2
Samantha Stosur (AUS) bt Angelique Kerber (GER) 7-6 (7/5), 7-6 (7/4)
Danielle Collins (USA) bt Elina Svitolina (UKR X4) 6-1, 6-1
Ajla Tomljanovic (AUS) bt Priscilla Hon (AUS) 6-2, 4-6, 6-4