World number one Aryna Sabalenka suffered a setback in her US Open build-up after she was beaten 7-6(3) 4-6 6-4 by Russia's Ekaterina Alexandrova in the fourth round of the Canadian Open in Toronto on Saturday.
The 16th-seeded Alexandrova fought a hard battle lasting nearly two and a half hours to secure her quarter-final berth.
Sabalenka, a four-time Grand Slam champion and reigning US Open winner, recovered from a set down to force a decider but was unable to halt Alexandrova's aggressive shot-making in the final set.
The Belarusian broke early in the opening set before Alexandrova rallied to force a tiebreak, which the Russian won to move ahead.
Sabalenka responded by seizing control of the second set with a break for a 4-3 lead before serving out to level the match.
Alexandrova held her nerve in the decider as Sabalenka's error count rose. Serving to stay in the match at 4-5, the top seed saved two match points, but a double fault on the third handed victory to the Russian.
"I just tried to play every single point as if it was the last one because with her, you don't (get) a lot of chances during the match," Alexandrova said in her on-court interview.
"Honestly, I was trying not to think about the score or anything. Just hit the ball, and that's it.
"I'm super happy that I could win because after the second set, I thought (the chance) was already past me."
Alexandrova will face Ukraine's Elina Svitolina, who defeated American eighth seed Amanda Anisimova 6-2 6-4, for a place in the semi-finals.
