Caroline Wozniacki retired from her semi-final match against Maria Kirilenko at the New Haven Open on Friday, a knee injury ending her bid for a fifth straight title in the US Open tune-up.

Wozniacki hurt her right knee in a quarter-final victory over Slovakia’s Dominika Cibulkova on Thursday.

She had lost the first set to Kirilenko 7-5 when she called for the trainer.

Wozniacki said she opted to play after doctors told her she probably wouldn’t make the injury worse.

But it troubled her in the first set and she decided to rest her leg for a couple of days in a bid to be ready for the US Open starting on Monday.

Wozniacki suffered her first match defeat at New Haven, where she was the four-time defending champion.