Slovakia's Dominika Cibulkova shocked fifth-seeded Russian Elena Dementieva 6-4, 6-2 to reach the Montreal Cup third round on Tuesday.
Dementieva was joined at the exit door by eighth-seeded compatriot Vera Zvonareva, who was upset 6-3, 1-6, 7-6 by Frenchwoman Virginie Razzano.
Japan's Ai Sugiyama provided the day's other upset, dispatching 16th-seeded Czech Nicole Vaidisova 6-3, 3-6, 6-2.
Russian fourth seed Svetlana Kuznetsova progressed, however, reaching the third round when compatriot Alla Kudryavtseva retired with a shoulder injury.
Kuznetsova, the 2004 US Open champion, took the opening set 6-1 and was 1-0 up in the second when Kudryavtseva, unable to serve, walked to net to shake hands.
"There's not much to talk about, I was attacking, put solid pressure on her second serve," said Kuznetsova, who has reached three finals but is still chasing her first title of the season.
"She made many double faults."
Belarusian 11th seed Victoria Azarenka also had an untroubled opener, pounding American Jamea Jackson 6-1, 6-1 and Italian 15th seed Flavia Pennetta rallied past Ukraine's Olga Savchuk 1-6, 7-5, 6-3.
Aleksandra Wozniak, who became the first Canadian in over 20 years to win a WTA singles title with victory at Stanford two weeks ago, delighted a capacity crowd with a 6-2, 1-6, 7-5 win over 18-year-old Chan Yung-Jan of Chinese Taipei.
The world's top three players, Serbian number one Ana Ivanovic, compatriot Jelena Jankovic and Russian Maria Sharapova begin their campaigns on Wednesday following first-round byes.