Caroline Wozniacki's first match on a grass court in five years ended in defeat by Elise Mertens at the Birmingham Classic.
Former world number one Wozniacki, Australian Open champion in 2018, lost 6-4 6-1 to Belgian fifth seed Mertens.
Denmark's Wozniacki, 33, came out of retirement last year, three-and-a-half years after ending her career at the 2020 Australian Open.
In Berlin, four-time major winner Naomi Osaka lost 6-4 3-6 6-3 to Chinese sixth seed Zheng Qinwen.
Britons Katie Boulter, Harriet Dart, Heather Watson and Amelia Rajecki are among the British players in action in Birmingham later on Tuesday.