World No. 1 Iga Swiatek was stunned by Beatriz Haddad Maia 6-4, 3-6, 7-5 on Thursday at the National Bank Open.
The unseeded Haddad Maia had already upset 13th-seeded Leylah Fernandez of Laval, Que., the previous day.
She will face the winner of the Round of 16 match between Belinda Bencic and Garbine Muguruza in a quarterfinal on Friday.
Serving was an issue for Swiatek with nine double faults to Haddad Maia's one.
Haddad Maia, ranked No. 24 in the world heading into the tournament, won the match point when Swiatek put a forehand long and wide.
Bianca Andreescu of Mississauga, Ont., who won the Canadian Open the last time the women played in Toronto in 2019, was scheduled to kick off Centre Court's evening slate with a match against China's Zheng Qinwen.
Serving for the match, the 10th-seeded American put the contest away on her first match point when Sabalenka returned Gauff's serve long.
Sabalenka, the sixth seed from Belarus, tossed her racket in frustration after surrendering the point in a marathon match that took three hours 11 minutes to complete.
Both players struggled with windy conditions at Sobeys Stadium, with Sabalenka committing 18 double faults and Gauff hitting into 15. Sabalenka had 42 unforced errors overall, while Gauff had 32.
Sabalenka and Gauff both struggled with their service game as the wind swirled before a packed house at the venue's grandstand, though both were able to hold through the first 10 games.
Gauff will face Romania's Simona Halep in the quarterfinals. Halep, a two-time Canadian Open champion, defeated Switzerland's Jil Teichmann 6-2, 7-5 to begin the day's slate of matches at Centre Court.
Later, seventh seed Jessica Pegula of the United States came back from a set down to defeat defending champion Camila Giorgi of Italy 3-6, 6-0, 7-5.
Pegula saved match point to tie the third set 5-5, then broke to take the lead on Giorgi's sixth double fault of the match. Pegula served to love in the final game to move on to the quarterfinals.