World No. 1 and defending champion Caroline Wozniacki lost to Italian Roberta Vinci in the second round at the $2.05 million Rogers Cup, a U.S. Open Series event.

Vinci, a three-time champion on the WTA circuit this season, ousted the Danish Wozniacki in 6-4, 7-5 fashion in 1 hour, 52 minutes Wednesday on the hardcourts at Rexall Centre.

Wozniacki beat U.S. Open runner-up Vera Zvonareva in last year's Rogers Cup finale in Montreal, as this event shifts between Toronto and Montreal each year.

French Open champion and Australian Open runner-up Li Na, seeded sixth this week, advanced via walkover, as fellow Chinese Peng Shuai was unable to go due to a hip injury, while seventh-seeded Wimbledon titlist Petra Kvitova of the Czech Republic handled Spaniard Anabel Medina Garrigues 7-6 (7-3), 6-3 and eighth-seeded French Open runner-up Francesca Schiavone of Italy leveled Russian Ekaterina Makarova 6-1, 6-3.

Last week's Carlsbad champion Agnieszka Radwanska, seeded 13th this week, continued her winning ways with a 6-3, 6-3 victory over Croatian qualifier Petra Martic on Wednesday.

In other second-round action, Czech Lucia Safarova subdued Ukrainian qualifier Simona Halep 6-2, 6-4 and Spanish qualifier Maria Jose Martinez Sanchez downed Slovakian Daniela Hantuchova 7-5, 6-2.

The 2011 Rogers Cup champion will earn $360,000.