Caroline Wozniacki's New Haven winning streak ended at 20 Friday when the four-time defending champion retired from her semifinal match with Maria Kirilenko because of a knee injury.

Wozniacki, who injured the right knee in her quarterfinal win Thursday, called for the trainer right after dropping the first set 7-5 to the 25-year-old Russian.

"I could feel it from the start," she said. "But, you know, it just started to get a bit worse. I decided to stop because if you don't feel a hundred percent, you can't compete at a hundred percent. It's better to let it rest. I wasn't going to win this match anyways if I'm not on a hundred percent fit level."

Kirilenko, the seventh seed, will face second-seeded Petra Kvitova on Saturday. The 2011 Wimbledon champion beat Italy's Sara Errani, 6-1, 6-3 on Friday night.

Using a serve that hovered at about 108 mph, and powerful returns, Kvitova needed just 24 minutes to win her first set. She then broke Errani in the first game of the second set and cruised from there, closing the match out with another service break.

"She serve more than me, she hit more than me," Errani said. "(It) was like for me inside like, 'No way.' "

Kvitova has already clinched the U.S. Open series championship, winning her first title of the year in Montreal and reaching the semifinals of Cincinnati before her run here. She's 41-11 in matches this year.

"The run that I have here in North America, it's the first one," she said. "I didn't have a great result before, so I'm really glad for it."

The third-seeded Wozniacki had won every match she played at the tournament since entering as a relatively unknown 18-year-old in 2008. She hadn't dropped a set here since the 2010 final.

Wozniacki was trying to join Martina Navratilova, Steffi Graf and Chris Evert as the only players to win five consecutive championships at the same tournament.

"You know, you defend it again and again, all of a sudden you're in this elite group that has won a tournament four times," she said. "I mean, there's a lot of players that never win a tournament and there's a lot of players that never win four. Winning the same one four times in a row is definitely special."

That was part of the reason she decided to play Friday, despite the injury, which she suffered on a backhand return in the opening game of the second set Thursday against Dominika Cibulkova. She went on to win that match 6-2, 6-1 after getting the knee taped.

She said medical experts had told her she would not hurt the knee further by playing Friday.

She seemed to move well during the match, but had a hard time with Kirilenko's serve and did not get single break point in the set.

Kirilenko broke Wozniacki in the 11th game and served out the set to avoid a tie-breaker. "She was fighting," Kirilenko said. "She was running. When I hit a great shot, she couldn't run for it. She made the right decision. She has to take a rest before the U.S. Open."

The Olympic doubles bronze medalist will be playing in her first final in the United States on Saturday, and seeking her first title of the year. She is hoping to duplicate her performance from Olympic quarterfinals, where she beat Kvitova 7-6 (3), 6-3.

"Every match I feel that I'm improving," Kirilenko said. "I found the way to play again on hard court, as grass court is different completely than hard. Today I played, I think, the best tennis in this tournament."