Wimbledon champion Petra Kvitova suffered a 6-3 7-6 (7-2) defeat by American Madison Brengle on her return to the WTA Tour in Stuttgart.
Kvitova, 25, was playing her first WTA match since February following a six-week break for exhaustion.
The Czech did win both matches on Fed Cup duty against France last weekend but was error-strewn in Stuttgart.
Top seed Maria Sharapova takes on in-form Angelique Kerber of Germany later on Thursday.
Denmark's Caroline Wozniacki, seeded fourth, plays Czech Lucie Safarova.
Third seed Kvitova made 22 unforced errors in the first set as the far steadier Brengle broke serve once but created six break points.