NEW YORK -- History repeated itself for Serena Williams, and not in a good way.
For the second straight year, Williams was stunned in the U.S. Open semifinals 6-2, 7-6 (5) by a seemingly lesser opponent, this time hampered by a knee injury as she double-faulted on match point against No. 10 Karolina Pliskova of the Czech Republic in Arthur Ashe Stadium on Thursday.
Pliskova takes an 11-match winning streak into Saturday's final against new world No. 1 Angelique Kerber of Germany, who dispatched unseeded Caroline Wozniacki, 6-4, 6-3 in the second semifinal. Included in that winning streak was a straight-set victory over Kerber last month in the Cincinnati final.
"I'm really happy to be there and even have the chance to play in the final here," said Pliskova, who took out Venus and Serena Williams en route to her first Grand Slam final. "It doesn't happen often that you're playing especially semifinal against Serena on center court here in New York. It doesn't happen often that you're in the final of Grand Slams."
Kerber, who owns a 4-3 career record against Pliskova, has a chance to win her second Grand Slam of the year, having beaten Serena in the Australian Open final before falling to Williams in the Wimbledon final.
"I will try to take the revenge against (Pliskova) again," Kerber said. "She has a big serve. She had a great four weeks in a row...I will go out here to enjoy my first final here and try to win the match, of course."
Williams said her knee began bothering her after "the second or third round." She said her mind was on the injury, but won't blame the loss on it.
"Yeah, I have been having some serious left knee problems," she said. "I wasn't able to move the way I wanted to move. When you're injured you're thinking of other things when you should be just playing and thinking of your shots. My mind was just a little bit everywhere. But it was what it was.
"I think I wasn't at 100 percent, but I also think she played well. She deserved to win today."
Pliskova was asked if it bothered her that Serena and her coach both mentioned the injury.
"If someone is not ready and she doesn't think she is ready, she should not go on the court," she said. "But after the match I would say it's still her choice if she wants to play or if she is ready to play or not.
"I'm just happy with my win, and I'm not gonna think about something like this."
Williams also surrendered her No. 1 world ranking, which she had held for a record-tying 186 consecutive weeks with Steffi Graf.
Kerber, 28, becomes the oldest first-time No. 1 player since 25-year-old Jennifer Capriati in 2000 and the first left-handed player to reach No. 1 since Monica Seles in 1996. Also, Kerber becomes the second German to earn No. 1 after Graf.
"For me it's just amazing to be after Steffi the next No. 1 player in Germany," Kerber said. "Steffi's a great champion, a great person. I think she's proud of me to be the next No. 1 after her."
A year ago, Serena's bid for a calendar Grand Slam was halted by Italy's Roberta Vinci in the U.S. Open semifinals.
This year, Williams was seeking to win her 23rd Open-Era Grand Slam title, which would have passed Graf's 22.
Pliskova became the fourth woman to beat the Williams sisters in the same Grand Slam event, having taken out Venus in the fourth round, which she said prepared her for the Serena match.
"There is not more than to beat both sisters in one tournament in New York," she said. "I'm really excited about those two wins. For the crowd, it's not probably the best that I beat both of them in their country, but for me it's really something special."
The loss was devastating for Williams, who was also seeking to break Chris Evert's record of six U.S. Open titles. She had already lost in the Australian and French Open finals, although she did win Wimbledon.
The big-serving Pliskova broke Williams twice in the first set and banged out a 112-mph service winner on set point to take the opening set in a cool 26 minutes.
Pliskova broke Williams in the fifth game of the second set to take a 3-2 lead when Williams sailed a backhand deep off a rally.
Williams broke back and consolidated for a 4-3 lead, shouting "Come on" when Pliskova sent a backhand long.
In the second semifinal, Kerber dispatched Wozniacki in relatively straightforward fashion, earning two breaks in the first set and getting another in the first game of the second set.
On match point, a replay showed that Kerber's final shot was beyond the baseline, but Wozniacki did not ask for a review and instead walked off the court.