Top seed Serena Williams avoided the upset bug at the Toronto Masters on Thursday, defeating Lucie Safarova 7-5 6-4 to reach the quarter-finals.

Williams fired 11 aces and won 76 per cent of her first serve points against the Czech in the 90 minute match on centre court in Montreal.

The American world No.1 next faces former world No.1 Caroline Wozniacki, who beat Shelby Rogers earlier Thursday 6-1 6-0.

While Williams progressed on cue, second-seeded Wimbledon champion Petra Kvitova and French Open winner Maria Sharapova both lost in the third round on Thursday.

World No.4 Kvitova lost 6-4 1-6 6-2 to Russia’s Ekaterina Makarova, while Sharapova fell 6-2 4-6 6-2 to Spain’s Carla Suarez Navarro.