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.

“Unfortunately my run for a 5th straight title here in New Haven came to an end,” Wozniacki said on Twitter. “Had to retire because of a knee injury … Some treatment and ice and I should be all good for the open next week!”

In Saturday’s final Kirilenko will face the winner of the later semi-final between second-seeded Petra Kvitova and fourth-seeded Sara Errani.

Kvitova, of the Czech Republic, has already secured the top spot in the US Open Series standings thanks to her performance in the series events leading up to the final Grand Slam of the season.

That gives her a chance to claim a $US1 million ($A961,000) bonus with a victory at Flushing Meadows.