Madison Keys' withdrawal due to a left wrist injury on Wednesday has left the top half of the Coupe Banque Nationale draw wide open.
Keys was the top seed at the WTA International event played on carpet, but a post on Twitter revealed she injured herself in training before her scheduled Quebec City opener against Anna Tatishvili.
Tatishvili went on to account for lucky loser Nadiia Kichenok 6-2 6-4, and the American - ranked 102nd - and Jelena Ostapenko (101st) are the highest-ranked players remaining in the top half of the 32-player draw.
Paula Kania is into the last eight, after a 6-3 6-3 victory over Sesil Karatantcheva - the Pole to play Latvia's Ostapenko for a spot in the semi-finals.
Ostapenko advanced past American qualifier Jessica Pegula 3-6 6-2 7-6 (7-3).
After coming from a set and 5-2 down to beat Nicole Gibbs, it was Pegula's turn to cough up a lead - she had a 4-1 double-break lead in the third set before Ostapenko fought back.
Fifth seed Annika Beck advanced into the quarter-finals with a 6-4 6-4 win over Andrea Hlavackova.