Karolina Pliskova's Stuttgart Open title defence fell at the first hurdle on Thursday as she lost to Victoria Azarenka.
Pliskova had won in Germany 12 months ago and, as fourth seed, received a bye through the first round this time, but she let an early lead slip in the late match on Centre Court.
The Czech star claimed the first set but was pegged back in the second, before a tense third looked to be going the distance until Azarenka nudged in front.
Azarenka's 4-6 6-3 6-4 triumph sees her book a date with Anett Kontaveit. The eighth seed beat Anastasia Pavlyuchenkova.
Anastasija Sevastova came out on top in a break-heavy match with Laura Siegemund 6-4 6-3, meaning she plays Petra Kvitova next.
Kvitova can no longer become world number one this week, however, after the top-ranked Naomi Osaka defeated Hsieh Su-wei, setting up a clash with Donna Vekic.
Vekic saw off Daria Kasatkina in straight sets.
Angelique Kerber and Kiki Bertens are set to meet on Friday after downing Andrea Petkovic and Belinda Bencic respectively in the last 16.
At the Istanbul Cup, Petra Martic battled back from a set down to defeat Ana Bogdan 4-6 7-5 6-3.
Kristina Mladenovic, Marketa Vondrousova and Veronika Kudermetova all also advanced in Turkey on Thursday.