Belinda Bencic and Lucie Safarova were sent packing from the Volvo Car Open in the second round on a mixed day for seeds in Charleston.
Swiss second seed Bencic and fourth-seeded Czech Safarova were bundled out in straight sets by Elena Vesnina and wildcard Louisa Chirico on Wednesday.
A finalist at the St Petersburg Ladies Trophy in February, Bencic was no match for Vesnina, who eased to a 6-1 6-1 victory in less than an hour at the WTA tournament.
Bencic failed to convert either of her two break points, tallied three double faults and won just 54 per cent of first serves.
Safarova - the 2012 runner-up - did not fare much better, swept aside by American teenager Chirico 6-3 6-3.
Chirico had a 100 per cent conversion rate on break points, having claimed all four offered in arguably the biggest win of her career.
Last year's runner-up and eighth seed Madison Keys was also eliminated, outlasted 6-7 (3-7) 6-4 6-4 by Laura Siegemund.
As for Monica Puig, she rallied from 5-2 down in the first set to stun sixth seed Andrea Petkovic 7-5 6-2.
Venus Williams, however, advanced to the third round in straight sets.
The 2004 champion and third seed saw off fellow American Alison Riske 6-4 6-2.
Seeds Sara Errani, Sloane Stephens, Samantha Stosur, Daria Gavrilova and Daria Kasatkina were also second-round winners.
Errani accounted for Yaroslava Shvedova 6-0 6-3, Stephens defeated Danka Kovinic 6-4 6-3, 2010 winner Stosur saw off Jana Cepelova 6-4 7-5, Gavrilova was too good for Zhang Shuai 6-4 6-0 and Ana Konjuh went down to Kasatkina 6-1 6-1.
Sabine Lisicki, the 2009 winner and 15th seed, suffered a 7-5 4-6 6-3 loss to Yulia Putintseva.
Eugenie Bouchard withdrew from her match with Lourdes Dominguez Lino due to a lower abdomen problem, trailing 6-4 1-6 1-0.
Meanwhile, Kristina Kucova prevailed 0-6 6-3 6-3 against Kateryna Bondarenko and Irina-Camelia Begu triumphed 6-4 4-6 6-2 over Bethanie Mattek-Sands.