Top seed Halep, a beaten finalist at the French Open earlier this month, looked to be on her way to victory after claiming the first set 7-5 against Annika Beck.
But, with Beck ahead 3-2 in the second, the Romanian opted to pull out because of shoulder problems.
Fourth seeded Spaniard Suarez Navarro joined Halep on the list casualties, withdrawing from her clash with China's Zheng Jie, who had been leading 7-5 0-1.
Zheng will face Elina Svitolina in the quarter-finals after the Ukrainian knocked out sixth seed Kirsten Flipkens of Belgium 7-6 (7-4) 6-4.
Svitolina claimed a frenetic first set on a tie-break before coming out on the right side of a second that featured five breaks of serve to ensure progression.
The one seed that did enjoy success at the grass-court tournament in 's-Hertogenbosch on Wednesday was eighth seed Klara Koukalova, a 6-2 7-6 (8-6) winner over German qualifier Mona Barthel.
Czech Koukalova needed just one hour and 18 minutes to dispatch Barthel and set up a meeting with Yaroslava Shvedova following the Kazakh's 7-6 (8-6) 7-6 (7-4) win over Michaella Krajicek of the Netherlands.
Elsewhere in the draw, American Coco Vandeweghe benefited from a retirement as compatriot Vania King withdrew with Vandeweghe ahead 6-7 (6-8) 6-0 2-1.