Caroline Wozniacki retired from her semi-final match on Friday against Maria Kirilenko at the New Haven Open, with 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.
"It's definitely not the way I would have liked to have finished," said Wozniacki, who was trying to join Martina Navratilova, Steffi Graf and Chris Evert as the only players to win five straight titles at the same WTA event.
"You know, it's unfortunate," the Dane said. "But these things happen.
"It was a difficult decision to stop."
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 second-seeded Czech Petra Kvitova, who defeated fourth-seeded Sara Errani, 6-1 6-3.
Kvitova, who claimed her first title of 2012 with a victory over China's Li Na at Montreal on August 12, had 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 the former Wimbledon champion a chance to claim a $US1 million ($A961,000) bonus with a victory at Flushing Meadows.