BAD HOMBURG    -    Former world number one Caroline Wozniacki had to retire from her quarter-final match at the Bad Homburg Open on Thursday due to a knee injury while Paula Badosa was knocked out.Danish wildcard Wozniacki was facing third-seeded American Emma Navarro and had taken the opening set in which she fell and seemed to hurt her knee.The 33-year-old former Australian Open champion was losing 4-6 6-1 1-0 when she retired, with one eye on Wimbledon, which starts on Monday, where she also has a wildcard. Spain’s former world number two Badosa, also a wildcard, struggled with double faults and Russian Diana Shnaider’s superior second serve in the second set as she lost 6-3 7-6(3).Shnaider will face Navarro in the semi-finals.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 face Bulgarian Viktoriya Tomova or Russia’s Anna Blinkova, who were playing later on Thursday, in the semis.