Caroline Wozniacki remained undefeated in New Haven on Thursday, but her quest for a fifth consecutive title here may hinge on her right knee.

Wozniacki suffered an injury early in the second set of her 6-2, 6-1 quarterfinal win over Dominika Cibulkova of Slovakia. She was hopeful, but not certain, that she would be able to play in Friday's semifinal against Maria Kirilenko.

"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."

On the other side of the draw, Czech star Petra Kvitova clinched the U.S. Open Series title with her quarterfinal win over Lucie Safarova.

The world's fifth-ranked player had little trouble with her friend and countrywoman, recording 10 aces and no double-faults in a 6-3, 6-3 win.

The series championship, based on points earned in five U.S. hardcourt tournaments this summer, makes Kvitova eligible for a bonus of up to $1 million depending on her finish in next week's U.S. Open.

"I don't care about the money," she said. "I'm really proud that I'm champ. But, I mean, the money is not the important thing in the world and in the life. So I'm so glad that I played a lot of matches, good matches. I have great results. I'm here in the semifinal. That's important for me to have some confidence for the US Open."

She will play and Sara Errani of Italy, who beat Marion Bartoli of France 6-4, 6-2 in last quarterfinal. Errani jumped out to a 4-0 lead in the first set and hung on for the straight set win. She said the key was taking Bartoli off the baseline.

"She plays so straight, so strong, and I just tried to change the game to make her run," she said.

Wozniacki is now 20-0 in New Haven. A fifth straight title would tie her with Steffi Graf (Hamburg), Martina Navratilova (Chicago) and Chris Evert (Charleston) as the only players with that many consecutive championships at the same tournament.

But it was not clear Thursday night how seriously she was injured. She said she felt a sharp pain in her knee when she hit a backhand return in the first game of the second set. After Cibulkova double faulted on the next point, Wozniacki walked to the chair umpire, motioned to her knee, sat down and grabbed a bag of ice from a cooler.

"I decided straightaway 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."

Wozniacki had the knee taped and returned to the court, and continued to dominate. But she seemed to have a slight limp at times, flexed her leg between points and looked over to the coach's box several times seeking guidance. She left after the match to have the knee examined and receive treatment.

Cibulkova said she was distracted by the injury. She won that first game of the set, but dropped the next six.

"I was paying too much attention to her," Cibulkova said. "When you ask for a physio like this, I was serving, it was my game, it was during the game, it doesn't happen often. When you do it, it has to be serious. You really either cannot move or you cannot continue or something like that."

Kirilenko advanced over qualifier Olga Govortsova 6-1, 6-2 who was celebrating her 24th birthday.

It is the second New Haven semifinal in three years for the seventh-seeded Russian, who lost to Nadia Petrova in 2010.

She's had a relatively easy week this year, winning her first match 6-2, 6-2 over Shahar Peer, and advancing to the quarterfinals when German Mona Barthel withdrew with a stomach bug.

"Before US Open, I (don't want) to use too much energy," Kirilenko said. "So the first match was quite easy. Second one I didn't play, so I had couple days off. But I was practicing really hard."