Caroline Wozniacki exits with knee injury at Bad Homburg Open

Former world No. 1 Caroline Wozniacki had to retire from her quarterfinal match at the Bad Homburg Open on Thursday with a knee injury.

Wozniacki, 33, a wild-card entry, fell during the first set of her match against third-seeded American Emma Navarro and seemed to hurt her knee. Wozniacki won the set, but she was losing 4-6, 6-1, 1-0 when she retired.

She also is a wild-card entrant at Wimbledon, which starts Monday.

Spain's Paula Badosa , the former world No. 2 who is also a wild card at this event, struggled with double faults and lost 6-3, 7-6 (3) to Russian Diana Shnaider , whose superior second serve helped carry her into the semifinals vs. Navarro.

Croatia's Donna Vekic is also in the last four after receiving a walkover when Czech defending champion Katerina Siniakova pulled out due to a hip injury.

Vekic will next face Viktoriya Tomova , who beat Anna Blinkova 2-6, 6-1, 7-6 (9).

Field Level Media contributed to this report.