The top seeded Karolina Pliskova, the defending champion, eased into the third round of the Cincinnati Open with a comfortable 6-2 6-3 win over Russian Natalia Vikhlyantseva but two other seeded players fell by the wayside, namely Angelique Kerber (3) and Venus Williams (9).
Kerber lost 6-4 1-6 7-6(11) to Russian Ekaterina Makarova, while Williams was upset 6-3 2-6 6-2 by Australian qualifier Ashleigh Barty in their second-round matches.
Barty now faces former world number one Caroline Wozniacki, a 6-2 6-4 winner over Russia’s Elena Vesnina, in the last 16.
Last year Pliskova beat Kerber in the final to score the biggest victory of her career and prevent the German from ascending to world No.1.
Next up for the 25-year-old Czech is a third-round match against Italian Camila Giorgi, who beat Australian Daria Gavrilova 7-6(1) 5-7 6-3.
American 16th seed Madison Keys beat Russian Daria Kasatkina 6-2 6-1 to reach the last 16, where she will face Wimbledon champion Garbine Muguruza of Spain.
For the second week in a row, American Sloane Stephens defeated 14th seed Petra Kvitova, who was bothered by pain in her right thigh, to reach the third round with a 6-2 6-3 victory.
Up next for Stephens will be the on-form Ekaterina Makarova.