NEW HAVEN - 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.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.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.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.