Belinda Bencic and Marketa Vondrousova both pulled off shocks.
Defending champion Naomi Osaka crashed out of the BNP Paribas Open at Indian Wells in a straight-sets loss to Belinda Bencic.
The world number one was immediately on the back foot in her fourth-round match, as Bencic raced to a 5-1 lead in the first set.
Despite Osaka winning the next two games on the bounce, a third break of serve for Bencic helped give her the early advantage.
The Swiss world number 23 continued to harness her momentum in the second set, winning 88% of points on her first serve to prevail 6-3 6-1 after an hour and seven minutes on court.
She will now face Karolina Pliskova for a spot in the semi-finals, after the Czech outlasted Anett Kontaveit 7-6 (0) 4-6 6-2.
World number two Simona Halep suffered a similar fate to Osaka, falling 6-2 3-6 6-2 to unseeded teenager Marketa Vondrousova.
The 19-year-old Czech player, ranked 61, delivered her first career victory over top-10 ranked opposition.
In Tuesday??ôs opening match at Stadium 1, Spaniard Garbine Muguruza also pulled off a shock when she came from behind to defeat Dutch number seven seed Kiki Bertens 5-7 6-1 6-4.
Meanwhile, Venus Williams converted four break points to beat Mona Barthel 6-4 6-4, her fourth consecutive win against the German.
Canadian wildcard Bianca Vanessa Andreescu, 18, defeated Chinese player Qiang Wang 7-5 6-2 to secure her place in the last eight, while number six seed Elina Svitolina came through a tough battle against Australia??ôs Ashleigh Barty to win 7-6 (8) 5-7 6-4.
Number eight seed Angelique Kerber outlasted ninth seed Aryna Sabalenka, claiming a 6-1 4-6 6-4 win.