MONTREAL -- Serena Williams is through to the quarter-finals, but the US$2.44 million women's Rogers Cup is over for Maria Sharapova.

The top-seeded Williams waited out a brief rain delay and then defeated Lucie Safarova 7-5, 6-4. She is now 7-0 in her career against the Czech left-hander.

Carla Suarez Navarro upset fourth-seeded Sharapova 6-2, 4-6, 6-2 in a match delayed twice by rain.

And Wimbledon champion and second seed Petra Kvitova is also out after a 6-4, 1-6, 6-2 loss to Ekaterina Makarova in a battle of left-handers.

In the quarter-finals on Friday, Williams will face the red-hot Caroline Wozniacki, who routed American qualifier Shelby Rogers 6-1, 6-0.

Williams, who won last year in Toronto, is playing in Montreal for the first time since she retired with an injury in the final in 2000.

Her match with Safarova played out from the service line, with one break in each set going to Williams. In the last game of the second set, Safarova double faulted twice and the hit match point long.

Suarez Navarro, ranked 16th in the world, returned from the second delay to break Sharapova's service and then staved off a break point in the next game thanks to a lucky net cord. She broke again in the final game.

Sharapova said playing on centre court, which has more space on the sides and behind the baseline than a regular court, may have helped her Spanish opponent.

"I think that favoured her quite well as she was able to step so far behind the baseline and retrieve a lot of balls," said Sharapova.

Sharapova, just getting started on the hardcourt season, had struggled to a three-set win in her opening match Wednesday against Garbine Muguruza. She said she has much to work on in her game heading toward the U.S. Open at the end of August.

"I couldn't find my rhythm from the beginning of the match," said the 27-year-old, who won a fifth career grand slam title this year at the French Open. "I always had my back against the wall, always had to come back."

She said the rain, which sprinkled just enough to cause about 30-minute delays, was not a factor.

The 25-year-old Suarez Navarro won her first WTA tournament this year at the Portugal Open in May.

Earlier, Wozniacki downed Rogers to continue her run of one-sided victories.

The 11th seed from Denmark, coming off a victory in Istanbul, has dropped only six games in three matches en route to the quarter-finals.

"I felt really comfortable and confident," said Wozniak. "I served and returned well.

"She's a tough player. She has some big shots. I was just able to neutralize them and play my game."

Rogers reached the third round with an upset of local favourite Eugenie Bouchard on Tuesday.

Suarez Navarro will face the winner of an evening match between Venus Williams and Angelique Kerber. Makarova, rabked 19th in the world, wil play the winner between Jalena Jankovic and qualifier Coco Vandeweghe.