The fifth-seeded Sharapova succumbed to fellow Russian Galina Voskoboeva 6-3, 7-5 in 1 hour, 47 minutes on a blustery day on the hardcourts at Rexall Centre. The qualifier Voskoboeva broke Sharapova's serve five times en route to the upset.
The three-time Grand Slam champion Sharapova has now dropped three of her last five matches overall.
The third-seeded Zvonareva, meanwhile, gave way to hot Pole Agnieszka Radwanska, who won her eighth straight match by dousing her Russian counterpart in 6-4, 7-6 (7-4) fashion. The 13th-seeded Radwanska was last week's surprise titlist in Carlsbad.
Fourth-seeded Belarusian star Victoria Azarenka whipped Spanish qualifier Maria Jose Martinez Sanchez 6-1, 6-2 in 63 minutes on Day 4. The Wimbledon semifinalist Azarenka will battle Voskoboeva in Friday's quarterfinals.
A pair of 2011 Grand Slam champions exited the draw Thursday, as 10th-seeded Aussie Samantha Stosur doused sixth-seeded French Open champion and Aussie Open runner-up Li Na of China 6-2, 6-4 and 11th-seeded rising German Andrea Petkovic dismissed seventh-seeded Wimbledon winner Petra Kvitova of the Czech Republic 6-1, 6-2 in 73 minutes. This marked Li's first match of the week after she enjoyed an opening-round bye and a walkover in the second round.
Petkovic will tangle with Radwanska here on Friday.
Ever-improving Italian Roberta Vinci, a three-time WTA champion this season, drove out former world No. 1 Ana Ivanovic 7-6 (7-3), 6-2. The Serbian Ivanovic captured this Canadian event in Montreal back in 2006.
Vinci will face Stosur here on Friday.
The 2011 Rogers Cup champion will earn $360,000.