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.

British women's number one Boulter, who retained her Nottingham Open title on Sunday, starts against Ukraine's Anhelina Kalinina, while Dart meets Japan's Moyuka Uchijima.

Watson faces Czech eighth seed Marie Bouzkova, and 22-year-old Rajecki contests her first WTA main draw match against Russian Diana Shnaider.