Varvara Lepchenko suffered a shock loss at the Coupe Banque Nationale, while defending champion Oceane Dodin withdrew.
Lepchenko, the fifth seed at the WTA International event, was beaten by Canadian Francoise Abanda 5-7 7-5 6-4 in Quebec City on Wednesday.
Abanda, 20, needed two hours, 15 minutes to secure her surprise win, breaking twice in the deciding set.
Awaiting Abanda in the quarter-finals is Timea Babos after the Hungarian third seed got past Naomi Broady 6-2 5-7 6-2.
Dodin, the second seed and defending champion, withdrew from her second-round match with qualifier Caroline Dolehide, citing dizziness.
It handed American teenager Dolehide a passage into the last eight.
Meanwhile, fourth seed Tatjana Maria needed just 53 minutes to thrash Grace Min 6-1 6-1.