Aryna Sabalenka was knocked out of the Canadian Open by Russia's Ekaterina Alexandrova as the world No 1 suffered an early setback ahead of her US Open defence.
Sabalenka was beaten 7-6(3) 4-6 6-4 in the fourth round in Toronto with Alexandrova prevailing after nearly two and a half hours to reach the quarter-finals.
The Belarusian was playing in her first tournament since Wimbledon, where the four-time grand slam champion suffered a fourth-round defeat to Naomi Osaka.
Sabalenka has not won a tournament since winning Indian Wells and the Miami Open back-to-back in March, and has only reached one semi-final in her last six tournaments.
Before the US Open, Sabalenka faces the same situation as last season where she arrived in New York as the world No 1 but without a winning grand slam title this year.
Sabalenka defeated Amanda Anisimova in last year’s final to win her second US Open but her form 12 months on is more of a concern. The US Open starts later this month.
Alexandrova will face Ukraine's Elina Svitolina, who defeated American eighth seed Anisimova 6-2 6-4, for a place in the semi-finals.
Elsewhere, Iga Swiatek rallied from a set down and overcame struggles with her serve to defeat Marta Kostyuk 3-6 6-1 6-2.
Third seed Jessica Pegula, who was last week’s Washington finalist, lost 6-3 6-3 to Russia’s Diani Schnaider, who will next face Swiatek.
