Montreal - Defending champion Oceane Dodin of France withdrew from the WTA Tour's tournament in Quebec City on Wednesday complaining of dizziness.
The sudden departure of the second seed, ranked 48th in the world, saw American Caroline Dolehide, ranked 161st, secure a quarter-final berth by walkover.
Dodin began warming up about an hour before the scheduled start of her match, but soon decided she couldn't play.
Three other matches were completed, with third-seeded Timea Babos of Hungary battling past Naomi Broady of Britain 6-2, 5-7, 6-2.
Broady broke serve in the 12th game of the second set to force a deciding set, then won the first game of the third.
But Babos, who is ranked 63rd in the world, won three straight games to take charge.
Babos next faces Canadian Francoise Abanda, who rallied to beat fifth-seeded American Varvara Lepchenko 5-7, 7-5, 6-4.
Fourth-seeded German Tatjana Maria booked her quarter-final berth with a 6-1, 6-1 win over American Grace Min.