British number one Heather Watson suffered a 6-2 6-4 loss to eighth seed Victoria Azarenka of Belarus in the Rogers Cup third round in Montreal.
Guernsey's Watson, 22, was broken three times in the first set.
Ranked 57th in the world, she won three successive games in the second set before the ex-world number one took it 6-4 and now meets Agnieszka Radwanska.
There were victories for Serena and Venus Williams, but second seed Petra Kvitova lost to Ekaterina Makarova.
|Azarenka after beating Watson|
|"I felt like I started to be less dominant for a couple of games, but then I brought it back and stepped up when I needed to."|
Azarenka, who returned to action at Wimbledon after three months out with a foot injury, took her record against Watson to 4-0.
"I started really well and played pretty consistently throughout the whole match," said the Belarusian.
"She stepped up her game in the second set. I felt like I started to be less dominant for a couple of games, but then I brought it back and stepped up when I needed to."
Watson had enjoyed one of the best victories of her career in beating 12th seed Dominika Cibulkova in the previous round.
Her results in Toronto means she is set to return to the world's top 50 next week, having started the year ranked 119th following illness and injuries in 2013.
Russian Makarova claimed a 6-4 1-6 6-2 victory over Kvitova and will now play Coco Vandeweghe after the American beat seventh seed Jelena Jankovic of Serbia 7-6 (8-6) 2-6 7-5.
World number one Serena Williams defeated Czech Republic's Lucie Safarova 7-5 6-4 and unseeded Venus Williams overcame Germany's sixth seed Angelique Kerber 6-3 3-6 6-4.
Venus Williams will now play Carla Suarez Navarro after the Spaniard won 6-2 4-6 6-2 against Russia's Maria Sharapova, the French Open champion.