Maria Sharapova crashed out in the first round of the Kremlin Cup in Moscow on Tuesday, losing in straight sets to Slovakia's Magdalena Rybarikova.
Karolina Pliskova became the firm Wimbledon favourite on Wednesday when compatriot and two-time champion Petra Kvitova was knocked out.
Fourth-ranked Petra Kvitova, the 2012 New Haven winner who lost to Simona Halep in last year's final, will be the women's third seed when the year's last Grand Slam tournament US Open begins Monday in New York.
Fourth-ranked Petra Kvitova fired 11 aces in defeating Australia's Samantha Stosur 6-3, 6-1 while Magdalena Rybarikova hit seven aces in a 6-2, 6-4 triumph over Italy's Camila Giorgi at the US Open hardcourt warm-up event.
Simona Halep lost 2-6, 6-4, 3-6 against Slovakia's Magdalena Rybarikova in the second round of final US Open tune-up event.
The 20th-ranked Williams is playing in her first tournament since losing to Maria Sharapova in the third round of the Australian Open in January.
US top seed Andy Roddick, seeking his third Memphis crown and 30th ATP title, outlasted Australian Lleyton Hewitt 4-6, 6-3, 6-4 Friday to reach the semi-finals of an ATP and WTA indoor event.