Garbine Muguruza lost her third straight match, while Madison Keys bounced back to form at the Western and Southern Open on Monday.
The two highest-ranked players in action at the Western and Southern Open on Monday were beaten, with Wimbledon finalist Garbine Muguruza's struggles continuing.
Ninth seed Muguruza was ousted by qualifier Yaroslava Shvedova 6-4 7-6 (7-0), while 12th seed Timea Bacsinszky fell to Madison Keys 6-4 6-3.
Bacsinszky's task was always going to be a tough one against local and world number 19 Keys, who was looking to bounce back from a poor loss to Ajla Tomljanovic in Stanford earlier in August.
Muguruza was unable to turn around her form, however, making it two losses since her heroic run to the SW19 final.
A rain delay gave Muguruza a small reprieve when Shvedova led by a set and 3-2 - with a break - as the Spaniard broke back in the first game after the resumption.
All games went to serve thereafter, albeit Muguruza fended off three break chances in the 11th game, but Shvedova routed the tie-break to oust the world number eight.
Timea Babos joined Shvedova as a winning qualifier, edging former US Open champion Sam Stosur 7-6 (8-6) 4-6 7-5.
Stosur was serving to stay in the match at 4-5 when the rain hit, and the Australian managed to save a match point to stay alive - before Babos broke in the 12th game.
Not as fortunate were qualifiers Lucie Hradecka, Ana Konjuh or Yulia Putintseva.
Tsvetana Pironkova will be Serena Williams' first opponent at the WTA Premier hard-court event, after the Bulgarian defeated Hradecka 7-5 3-6 6-1.
Konjuh lost to Karin Knapp 7-5 6-1, while Putintseva was a first-round loser to wildcard Coco Vandeweghe 6-3 6-2 - with reports the pair re-ignited past grievances.
In 2013, Vandeweghe tweeted Putintseva displayed the "worst sportsmanship" during a match - after Monday's victory, the American said via ESPN: "The win speaks for itself. The scoreline speaks for itself. That's all I have to say."
Venus Williams set up a showdown with sixth seed Ana Ivanovic, after getting past Zarina Diyas 7-6 (8-6) 6-4.
Barbora Strycova blew a 40-0 lead when serving for the match, to slump to a 2-6 7-5 7-6 (8-6) loss to Varvara Lepchenko.
Lepchenko nearly repaid the favour when she let a 6-3 lead slip in the third-set tie-break, before winning on match point number four.
Former world number one Jelena Jankovic progressed with a 4-6 6-4 6-2 win over Madison Brengle.
Elina Svitolina saw off wildcard Alison Riske, the 14th seed winning 6-2 4-6 6-0, Irina-Camelia Begu ousted Alize Cornet 6-4 6-4 and 15th seed Andrea Petkovic overcame fellow German Julia Goerges 6-4 6-2.