Dementieva has not got a good recent record in Canada
Russian fifth seed Elena Dementieva was beaten in her first match at the WTA Rogers Cup in Montreal for the third consecutive time.
Dementieva lost to Slovakia's Dominika Cibulkova 6-4 6-2.
Fellow Russian Vera Zvonareva, seeded eight, was also a surprise loser, going out 6-3 1-6 7-6 to Virginie Razzano.
Britain's Anne Keothavong was knocked out in the first round by another Russian, Maria Kirilenko, beaten in straight sets 6-1 7-5.
Dementieva was playing her first match since her Wimbledon semi-final loss to eventual champion Venus Williams.
The 26-year-old had made it through to the final eight in her previous nine events since the Australian Open, including a title at Dubai.
But she struggled for form against Cibulkova, having her serve broken five times and winning only 47% of her service points.
Russian fourth seed Svetlana Kuznetsova advanced to the third round when compatriot Alla Kudryavtseva retired with Kuznetsova leading 6-1 1-0.
Kuznetsova is seeking her first title since New Haven last year.
Japan's Ai Sugiyama reached the second round by overcoming Czech 16th seed Nicole Vaidisova 6-3 3-6 6-2.
Russia's Nadia Petrova ousted Canada's Marie-Eve Pelletier 6-0 6-1 while Austrian Sybille Bammer defeated Australian Monique Adamczak 6-7 6-1 6-2.
Canada's Aleksandra Wozniak won her first-round match against Taiwan's Chan Yung-Jan 6-2 1-6 7-5 to book a second-round date against world number two Jelena Jankovic of Serbia.
Wozniak, ranked a career-best 41st, won her first title at Stanford earlier this month.