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.