Halep, the No.1 seed at the International-level grass court event, did a good job of rallying to win the first set - she was down 4-1 but managed to squeak it out, 7-5 - but down 3-2 in the second set the pain was just a little too much to handle. Beck advanced with a 5-7, 3-2 retirement victory.
The World No.3 is still hopeful for a good run at Wimbledon, where she'll be the No.3 seed.
"I think it was a good match today," she said after the match. "The injury isn't very dangerous, but I was cold on the court and the muscle at my shoulder and rib was contracted, and the physio told me I should stop playing, because it could have gotten worse. But I will be fine for Wimbledon."
It may not have come the way she wanted it, but Beck, a WTA Rising Star, scored her first Top 10 win.
Back injuries were something of an epidemic in 's-Hertogenbosch on Wednesday, unfortunately - No.4 seed Carla Suárez Navarro stopped with the same injury, handing Zheng Jie a 7-5, 0-1 victory, and Vania King the same, handing American qualifier CoCo Vandeweghe a 6-7(6), 6-0, 2-1 victory.
The three completed matches were all close encounters though, with No.8 seed Klara Koukalova holding off a late surge to close out German qualifier Mona Barthel, 6-2, 7-6(6), Elina Svitolina clinching a tight first set then rallying from 4-2 down in the second set to upset No.6 seed Kirsten Flipkens, 7-6(4), 6-4, and Yaroslava Shvedova fighting back from set point down at 5-4 in both sets to close out newly-engaged Dutch wildcard Michaella Krajicek in a pair of tie-breaks, 7-6(6), 7-6(4).
All four quarterfinals of the Topshelf Open will take place on Thursday, with No.7 seed Muguruza and No.8 seed Koukalova the only seeds remaining. Koukalova in particular holds a very special place in her heart for this venue - she won her first WTA title here all the way back in 2005, and when she did that she had been 0-5 in WTA finals (she beat fellow Czech Lucie Safarova in the final that year).