(Reuters) - Belgium's No.2 seed Elise Mertens and Dutch third seed Kiki Bertens both suffered surprise defeats in the Libema Open on Friday.
Mertens was beaten in the second round by German qualifier Antonia Lottner while her fellow Belgian Kirsten Flipkens defeated Bertens at the same stage.
Yet the U.S. top seed CoCo Vandeweghe battled into the semi-finals after surviving seven match points against compatriot Alison Riske.
Vandeweghe, a Wimbledon quarter-finalist in two of the last three seasons, finally prevailed 7-6(6) 3-6 7-6(12) after a marathon two hours and 56 minutes as she shoots for a third title at Hertogenbosch.
Mertens, who has won three titles this season, and Bertens both had their last-16 matches postponed to Friday after rain washed out play on Thursday.
Flipkens added to her 6-4 6-7(6) 6-1 victory over Bertens by beating Belarusian Aryna Sabalenka 6-4 6-4 a few hours later to book a spot in the last-four.
Lottner, however, failed to add to her shock 7-5 6-3 win over Mertens as she fell to a 6-3 6-2 defeat by 20-year-old Slovak Viktoria Kuzmova, who will face Flipkens in the semis.
Serbian Aleksandra Krunic ended Russian Veronika Kudermetova's surprise run with the seventh seed cruising to a 6-2 6-2 victory to set up a last-four clash against Vandeweghe.
(Reporting by Aditi Prakash in Bengaluru; Editing by Ian Chadband)
