Top seed Roberta Vinci was one of five seeds to fall in the second round of the Nurnberger Versicherungscup in Germany on Wednesday.
The Italian was beaten 6-4 7-6 (4) by Dutch qualifier Kiki Bertens, who took 79 minutes to secure her passage into the quarter-finals.
Second seed Laura Siegemund also bowed out, losing 7-6 (6) 3-6 6-1 to Colombian world number 98 Mariana Duque-Marino.
Fifth seed Sabine Lisicki was defeated 6-2 7-6 (5) by Varvara Lepchenko, while sixth seed Misaki Doi fell 6-3 5-7 6-4 to another American, Irina Falconi.
To complete a dismal day for the favoured players, eighth seed Yulia Putintseva lost 6-4 6-2 to home favourite Julia Goerges.
There was also an upset in the Internationaux de Strasbourg, where second seed Sloane Stephens crashed out 6-3 1-6 6-4 to French wild card Pauline Parmentier.
Fourth seed Kristina Mladenovic won a tight three-set fight with Alison Riske, while there were also wins for Virginie Razzano, Elena Vesnina and veteran Mirjana Lucic-Baroni.
Source: PA
