The 19-year-old Aussie came flying out of the blocks in Montreal on Wednesday (Thursday AEST), taking the first set, before the American bounced back strongly to claim a 4-6 6-4 6-3 victory.
Joint had been hoping to back up her first-round upset of local favourite Leylah Fernandez, the world No.24 who was fresh off claiming the biggest title of her career on Sunday in Washington.
Despite the loss, Joint has shown she is finding top form ahead of the year's final major, the US Open, which begins in New York on August 24.
Joint is already a two-time WTA title winner this year and will be looking to start making a real impact at grand slam level.
She reached the second round at Flushing Meadows in 2024, but is yet to progress past the first round of the other majors.
Australia's only remaining competitor in the women's singles, 15th-seeded Daria Kasatkina, plays her third-round match against 24th seed Marta Kostyuk of Ukraine on Thursday (local time).
In other second-round results, six-time grand slam champion Iga Swiatek was all business in a 6-3 6-1 demolition of China's Guo Hanyu.
The Polish superstar, seeded second in Montreal, is hot off her record-breaking win in the Wimbledon singles final, where she served up two "donuts" to American Amanda Anisimova.
"I think I played a really solid match today," Swiatek said.
"Playing the first match on hard court after a pretty long break from the surface is always tricky, so I'm happy that I just focused on myself, adjusted to the conditions and played well."
Fifth seed Anisimova enjoyed a 6-4 7-6 (7-5) success over Kiwi Lulu Sun and will next take on Brit Emma Raducanu, a 6-2 6-4 winner against Peyton Stearns.
"I'm really happy with how I came through in that match," said world No.33 Raducanu.
"I kind of lost my focus at some moments in the second set and she took advantage, but very happy that I came through against a top opponent and happy to extend my stay in Montreal."
Four-time major champ Naomi Osaka of Japan upset 13th-seeded Russian Liudmila Samsonova 4-6 7-6 (8-6) 6-3 to continue her impressive return after taking a maternity break.
American third seed and two-time defending champion Jessica Pegula remains on track for a title hat-trick by beating Maria Sakkari, of Greece, 7-5 6-4.
Sixth-seeded American Madison Keys crushed Germany's Laura Siegemund 6-2 6-1 and 20th seed Belinda Bencic of Switzerland saw off Canadian Eugenie Bouchard 6-2 3-6 6-4.
With PA and the AP.
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