Four-time defending champion Caroline Wozniacki advanced to the semi-finals of the WTA New Haven Open on Thursday but a slight right knee injury put a damper on the celebration.
"I decided straight away after I felt it," she said. "I played one more point and decided I needed to see the trainer because I felt a sharp pain in there. I needed to treat it."
The Danish third seed, who is set to be the eighth seed in next week's US Open, defeated Slovakia's Dominika Cibulkova 6-2, 6-1 in the quarter-finals and remained unbeaten at New Haven, improving to 20-0.
But Wozniacki felt a twinge in her right knee during the first game of the second set and called for a trainer.
After a Cibulkova double fault on the next point, Wozniacki walked to the chair umpire and pointed to her knee. She sat down and got a bag of ice from a cooler.
She continued with the knee wrapped and won with ease, improving to 8-3 all-time against Cibulkova, but planned to have the knee examined before deciding if she would play on Friday.
Wozniacki is scheduled to face Russian seventh seed Maria Kirilenko, who beat Olga Govortsova of Belarus 6-1, 6-2 in another quarter-final Thursday.
Wozniacki is the four-time defending champion and looking to become just the fourth woman to win the same WTA event in five consecutive years.
The former world number one played with a slight limp at times on Thursday and flexed her leg between points.
"You need to judge whether you can continue or you cannot," she said. "You need to judge whether it's going to get worse or it's not. That's all decisions you need to make yourself, decisions that are important because you don't want to jeopardize long-term injury. Doesn't matter what tournament it is."
It is the second New Haven semi-final in three years for Kirilenko.
In the other half of the draw, second-seeded Petra Kvitova swept past Lucie Safarova, 6-3, 6-3.