Andrea Petkovic battled back to advance to the semi-finals at the Family Circle Cup in Charleston, South Carolina on Friday while Czech qualifier Lucie Hradecka upset yet another seed with a victory over Sara Errani.
Last year's champion Petkovic had to rally from a set down to overcome qualifier Danka Kovinic 2-6, 6-3, 6-1 to set up a semi-final against fifth-seeded fellow German Angelique Kerber, who beat Romania's Irina-Camelia Begu 7-6, 7-6.
"She played really well in the first set," the third-seeded Petkovic said in a courtside interview.
"She has a tremendous serve for her height. It's really one of the best serves that I had to return on the WTA tour."
Hradecka's victory continued a string of upsets at the tournament, which has had several early exits of top seeds including Canada's Eugenie Bouchard and Russian Ekaterina Makarova.
A decorated doubles player, but ranked only 110th in singles, Hradecka beat fourth seed Errani 6-2, 6-4, her third victory against a seeded player on her way to the semi-finals.
Hradecka will face seventh-seed Madison Keys, who overpowered qualifier Lauren Davis 6-2, 6-2.
Image: Andrea Petkovic
Photograph: Clive Brunskill/Getty Images