Defending champion Anett Kontaveit suffered a shock first-round defeat at the Rosmalen Grass Court Championships at the hands of Veronika Kudermetova.
Qualifier Kudermetova claimed a second top-30 win in as many tournaments. She knocked out Carla Suarez Navarro after qualifying in Stuttgart in April, and pulled off a stunning 6-4 7-6 (7-4) triumph over fourth seed Kontaveit on Tuesday.
It marks the second successive year that the defending champion in Hertogenbosch has tumbled out in the first round, Coco Vandeweghe having lost to Carina Witthoeft in 2017.
Fifth seed Shuai Zhang also fell at the first hurdle, beaten by qualifier Fanny Stollar, but third seed Kiki Bertens came from a set down to defeat Natalia Vikhlyantseva and second seed Elise Mertens defeated Polona Hercog in straight sets.
Two more seeds, Aleksandra Krunic and Aryna Sabalenka, prevailed, and there were also wins for Alison Riske, Belinda Bencic, Ajla Tomljanovic, Kirsten Flipkens, Viktoria Kuzmova and Antonia Lottner.
Veronika Kudermetova upsets defending champ Anett Kontaveit!— WTA (@WTA) June 12, 2018
Sealing a straight sets win 6-4 7-6 (7-4) pic.twitter.com/eLhtR0fXtP
Meanwhile, at the Nottingham Open, top seed Ashleigh Barty - beaten in the second round by Serena Williams at the French Open - started her grass court campaign with a 6-3 6-3 success against Stefanie Voegele.
The Australian said: "It was pretty clean for a first match on grass. I'm very pleased with that."
Fourth seed Johanna Konta eased past Kurumi Nara 6-2 6-3 on a good day for the home players, with Britons Heather Watson and Katie Boulter also emerging victorious, though qualifier Katie Swan lost to Mona Barthel.
Mihaela Buzarnescu and Donna Vekic each won in three sets, but seventh seed Zarina Diyas had to retire from her match with Samantha Stosur after injuring her knee following a heavy fall.