The Danish wildcard Wozniacki was facing third-seeded American Emma Navarro on Thursday and had taken the opening set in which she fell and seemed to hurt her knee.
The 33-year-old former Australian Open champion, who is making a comeback to the sport after a break in which she gave birth to two children, 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 No.2 Paula 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 (7-3).
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 play Bulgarian Viktoriya Tomova, who saved five match points in the final set tiebreak to edge Russia's Anna Blinkova 2-6 6-1 7-6 (11-9) later on Thursday, in the semis.
In Eastbourne, British stars Katie Boulter and Emma Raducanu were denied another all-UK semi-final showdown ahead of Wimbledon after comprehensive defeats.
British No.1 Boulter, who registered a last-four win over Raducanu 11 days ago en route to retaining the Nottingham Open title, slipped to a 6-1 7-6 (7-0) loss against French Open finalist Jasmine Paolini.
Former US Open champion Raducanu was then comfortably dispatched 6-2 6-2 by last year's beaten finalist Daria Kasatkina.
17°C