The top seed crashed out 2-6 6-3 7-5 to her Belgian opponent who, at number 102 in the world, is ranked 89 places below her.
The Slovakian's match completed the first round of matches in the Canadian city, but there was more drama in the second round with second seed Yanina Wickmayer also sent packing.
The Belgian world number 27 was thumped 6-1 6-1 by American Lauren Davis.
Third seed Mona Barthel, who kicked off her campaign with victory over Britain's Heather Watson earlier in the week, is the highest remaining seed after she eased into the quarter-finals with a 6-2 6-1win over Slovenia's Petra Rampre.
Barthel will now meet defending champion and sixth seed Barbora Zahlavova Strycova of the Czech Republic, who advanced with a regulation 6-3 6-3 win over Germany's Tatjana Malek, for a place in the semis.
- Yanina Wickmayer
- Mona Barthel