Andreescu beaten in Toronto opener as qualifier Wang causes upset
Tsurenko, ranked 85th, rallied to defeat Andreescu 0-6, 6-3, 6-4 in just under two hours to book a second-round match against 10th seed Anna Kalinskaya.
There was some good news for Andreescu, who received a wildcard entry into next week's Cincinnati Masters.
Chinese qualifier Wang Yafan defeated American Sofia Kenin, the 2020 Australian Open winner, 7-5, 7-6 (7/3).
Wang will face top seed and reigning US Open champion Coco Gauff in the second round.
Ukraine's Elina Svitolina, the 2017 Canadian champion, won the first nine games in ousting American Shelby Rogers 6-0, 6-2 in 70 minutes.
Svitolina will next face 11th-seeded compatriot Marta Kostyuk, who eliminated Germany's Tatjana Maria 6-0, 6-2.
Former world number one Victoria Azarenka defeated Anastasia Pavlyuchenkova 6-2, 6-2 in 76 minutes, giving her a WTA-best 204h Masters 1000 match victory.
Poland's Magdalena Frech rallied to win five of the last six games of the second set and sweep the third to defeat Anastasia Potapova 4-6, 6-3, 6-0.
