Sabalenka lost 7-6, 4-6, 6-4 to the world no 11 to continue her bizarre 2026 season, which has included plenty of early exits from major tournaments.
"I just tried to play every single point as if it was the last one because with her, you don't a lot of chances during the match," Alexandrova said in her on-court interview.
"I had a lot today, but I couldn't realise some of them. But 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."
The star has claimed three WTA titles this year, including the Sunshine Double, but she has also been beaten before the semi-final stage at Wimbledon, the Italian Open, the Madrid Open, and Roland Garros.
It's becoming a worrying trend for Sabalenka, who has been very consistent over the last two seasons, in which she has held the world No 1 spot very comfortably.
One positive for Sabalenka after her Canadian Open defeat is she will continue to be the world No 1 no matter what happens at the Toronto event.
Elena Rybakina, who is closest to Sabalenka as the world No 2, is becoming close to usurping the Belarusian star, but it won't happen even if she wins the Canadian Open.
Should Rybakina claim the Canadian Open, which she is the favourite to do so now, she will be just four points behind Sabalenka in the WTA Tour rankings.
Rybakina plays Liudmila Samsonova in the round of 16 and she has been buoyed by several early exits at the Canadian Open so far.
Elsewhere, ninth seed Elina Svitolina eased past American Amanda Anisimova 6-2, 6-4 to reach the last eight.