Sharapova, the Russian fourth seed, slumped to a 6-2 4-6 6-2 loss to Spain's Carla Suarez Navarro on the hard courts in Montreal.
The 14th-seeded Suarez Navarro broke Sharapova's serve nine times – three of those came in the deciding set – in the third-round win.
Suarez Navarro faces another powerful opponent in the quarter-finals in American Venus Williams, who also claimed a scalp by beating German sixth seed Angelique Kerber 6-3 3-6 6-4.
Czech second seed Kvitova exited the WTA Premier event after a 6-4 1-6 6-2 defeat to Russian left-hander Ekaterina Makarova.
On a day of upsets, Makarova's quarter-final opponent Coco Vandeweghe stunned Jelena Jankovic.
American qualifier Vandeweghe fought to a 7-6 (10-8) 2-6 7-5 win over the Serbian seventh seed.
As seeds around her were beaten, Serena Williams avoided problems against Czech Lucie Safarova.
The world number one clinched a 7-5 6-4 victory over the 15th-seeded Safarova.
Williams will face in-form Danish 11th seed Caroline Wozniacki, who needed just 56 minutes to crush American qualifier Shelby Rogers 6-1 6-0.
Rogers could win just four points in the second set hammering.
Belarusian eighth seed Victoria Azarenka was a 6-2 6-4 winner over British qualifier Heather Watson and Polish third seed Agnieszka Radwanska overcame German Sabine Lisicki 6-1 3-6 6-3.