Russian Ekaterina Makarova upended second-seeded left-hander Kvitova 6-4, 1-6, 6-2, while diminutive Spaniard Carla Suarez Navarro stunned fourth-seeded Sharapova 6-2, 4-6, 6-2, following a third-set rain delay, at Uniprix Stadium.
Williams advanced by defeating Lucie Safarova 7-5, 6-4 to reach the quarter-finals.
Suarez Navarro, who is seeded 14th in Montreal and ranked 16th in the world, shrugged off two rain delays at the $2.4 million event and emerged with the victory over Sharapova who saw her preparations for the upcoming US Open suffer a huge setback.
The former world No. 1 Sharapova, who'd won 11 of her previous 12 matches on tour, has now lost her last two three-set matches after winning 11 straight.
Suarez Navarro put on a crisp shot-making display as she cruised through the first set.
Sharapova, who had also started slowly but managed to rally against second-round opponent Garbine Muguruza, battled back from a 4-2 deficit in the second to force the decisive third set, but she had no answer for Suarez Navarro in the third.
"Even though I felt like I started feeling a little bit better, it wasn't enough in the end.
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