The No. 1 seed has fallen at the National Bank Open presented by Rogers women's tournament.
Aryna Sabalenka of Belarus lost 7-6 (3), 4-6, 4-6 to Russia's Ekaterina Alexandrova in the Round of 16 on Saturday in Toronto.
Sabalenka hadn't dropped a set in either of her two opening wins.
Alexandrova will next face ninth-seeded Elina Svitolina, who beat No. 8 Amanda Anisimova 6-2, 6-4 later Saturday.
Sabalenka struggled with her serve in the loss, firing six aces to seven double-faults. Both players were broken on serve six times apiece.
Alexandrova finished with 109 points to Sabalenka's 104.
Sabalenka is the second high seed to fall on Saturday after No. 3 Jessica Pegula also exited with a loss to 15th-ranked Diana Shnaider.
