Despite tallying just one ace, Pera won 20 of 28 first-serve points (71.4 percent) and 14 of 18 points on the second serve (77.8 percent) to pull off the upset. She also committing just two double faults compared to Krejcikova's four.
Fellow American Madison Brengle (32) followed in Pera's footsteps, battling back to upset third-seeded Russian Ekaterina Alexandrova (27) 3-6, 7-5, 7-5. China's Shuai Zhang (33) also defeated a seeded opponent, but did so on a walkover, as Italian No. 2 seed Martina Trevisan (28) withdrew from their match due to a thigh injury.
In other action, Liudmila Samsanova (23) of Russia made quick work of Germany's Laura Siegemund (34) with a 6-2, 6-1 victory.
Championnats Banque Nationale de Granby
No. 10 seed Marta Kostyuk (20) of Ukraine rallied from a first-set loss to defeat Canadian Marina Stakusic (17) 6-7 (3), 6-4, 6-1 in round-of-16 action at Granby, Quebec.
Kostyuk avoided the upset by converting 7 of 13 break points (53.8 percent) and winning 55 of 92 service points (59.8 percent). After losing five serves across the first set and its tiebreaker, she only lost one for the remainder of the match.
Rebecca Marino (31) fared much better than her fellow Canadian, edging No. 3 seed Jasmine Paolini (26) of Italy 4-6, 6-1, 6-2. France's Diane Parry (19) and Germany's Tatjana Maria (35) also earned wins as underdogs, toppling eighth seed Tereza Martincova (27) of the Czech Republic and fourth seed Anna Bondar (25) of Hungary, respectively.
Australian ninth seed Daria Saville (28) swept Canada's Katherine Sebov (23) 6-1, 6-0, and China's Xiyu Wang (21) was a 7-6 (4), 6-1 winner over Cadence Brace (17) of Canada.
Also, No. 1 seed Daria Kasatkina (25) of Russia completed a 6-3, 6-2 victory over Poland's Magdalena Frech (24). Play was halted Tuesday due to rain.
