It was a tough day session for the top players in action at the National Bank Open presented by Rogers.
In the sweltering heat at IGA Stadium, two major seeds fell.
In a contest chock-full of ups and downs, No.5 seed Garbine Muguruza of Spain was toppled by Katerina Siniakova of Czechia (6-2, 0-6, 6-3).
As for no.9 seed Elise Mertens of Belgium, she was sent packing by Camila Giorgi of Italy after two sets (6-3, 7-5).
Earlier, 17-year-old phenom Coco Gauff had a relatively easy day at the office. She outplayed Anastasija Sevastova of Latvia (6-1, 6-4) to book her spot in the second round.
Sloane Stephens made the most of her main draw wild card and disposed of Dayana Yastremska of Ukraine (6-4, 1-6, 6-4) in a match that was suspenseful until the very last point.
British qualifier Johanna Konta found her way to the third round when Shuai Zhang of China was stopped in her tracks by an injury at 4-6, 5-2.
Jessica Pegula of the US secured a well-deserved win over Anett Kontaveit of Estonia (5-7, 6-3, 6-3), and Veronika Kudermetova of Russia outplayed Yulia Putintseva of Kazakhstan (1-6, 6-2, 6-4).
Finally, French qualifier Caroline Garcia lost to Anastasia Pavlyuchenkova (10) of Russia in two identical sets (6-4, 6-4).
Kvitova gains ground
In the last two matches of the day session, Petra Kvitova (7) of Czechia disposed of Fiona Ferro of France (6-4, 6-4) to advance to the third round. As for Elena Rybakina (12) of Kazakhstan, she found herself in a less favourable situation when she was knocked out by Liudmila Samsonova of Russia in three tight sets (6-4, 5-7, 6-4).
Sara Sorribes Tormo of Spain humbled Carol Zhao of Canada (6-2, 6-3), and Donna Vekic of Croatia ousted Alison Van Uytvanck of Belgium (6-1, 6-2).