TORONTO (Reuters) - Serena Williams took another step toward her fourth Rogers Cup title after dispatching Andrea Petkovic 6-3 6-2 on Thursday to advance to the quarter-finals.
Williams, who won the title in 2001, 2011 and 2013, dominated on service and blasted nine aces in a match that lasted just over an hour.
"I think I served better. That was the big difference," Williams told reporters. I still think I have a tremendous amount of improvement to do off the ground and everywhere else."
Williams will now take on Roberta Vinci of Italy, who beat Daria Gavrilova 6-4 6-3.
Second seed Simona Halep struggled to overcome a predictably tough examination from Angelique Kerber before prevailing 6-3 5-7 6-4.
Both players found it difficult to hold serve, with the world number three Halep breaking the 11th ranked German's delivery nine times, while the Romanian was broken eight times herself.
But Halep, runner-up at the French Open last year, did just enough to eliminate the 13th seed and advance to the quarter-finals against Agnieszka Radwanska, who beat France's Alize Cornet 6-2 4-6 6-2.
It was a bad day for German players, with Kerber's compatriots Sabine Lisicki and Carina Witthoeft also shown the exit door.
Swiss teenager Belinda Bencic beat Lisicki 6-1 1-6 7-3(3) in a battle between unseeded players that lasted more than two hours.
Ukrainian Lesia Tsurenko beat Witthoeft 6-3 6-4, while Italian Roberta Vinci took 78 minutes to dispose of Russian Daria Gavrilova by the same score.
Reporting by Andrew Both in Cary, North Carolina; Editing by Ian Chadband