Olympic silver medallist Marketa Vondrousova pulled off an upset in Chicago to beat beat No.4 seed Anastasia Pavlyuchenkova 6-3, 6-2.
It took 72 minutes for 2019 Roland Garros runner-up Vondrousova to see off her Russian opponent, winning 70 per cent of second service, something that gave her the edge as she converted six of her seven break points.
Vondrousova will meet No.16 seed Jil Teichmann of Switzerland for a place in the last eight.
Meanwhile, Victoria Azarenka battled past wild card Hailey Baptiste 6-2, 4-6, 6-3 in a thriller.
Azarenka cruised through the first set before Baptiste mounted a comeback witan. Strong forehand shot that unsettled the Belarusian, who hit four double faults as Baptiste took out the set.
And Baptiste started the stronger in the third, going g 3-0 up, before Azarenka pulled it back to level at 3-3 and then going on to win the match.
Azarenka will face former World No.1 Garbiñe Muguruza in the round of 16.