US Open 2016: Serena Williams upset by Karolina Pliskova, will lose No. 1 ranking

    Story highlights

    • Williams exits in straight sets
    • Match ends on double fault
    • Angelique Kerber to rise to No. 1
    • Kerber to face Pliskova in final

    (CNN)For the second straight year, Serena Williams was upset in the US Open semifinals.

    Whereas the American was defeated by crafty veteran Roberta Vinci 12 months ago to end hopes of a rare calendar-year slam, an ailing Williams lost to big-hitting Karolina Pliskova 6-2 7-6 (7-5) on Thursday in New York.
      She'll thus have to wait to become the first player in the Open Era to win 23 majors -- Williams is currently tied with Steffi Graf on 22.
      And in a double blow, when the new rankings are released Monday, the 34-year-old will lose top spot to Angelique Kerber, ending Williams' record-tying -- with Graf -- 186th straight week at No. 1.
      It certainly wasn't vintage Williams about 24 hours after she outlasted Simona Halep in a slugfest and she told reporters post match Thursday she was dealing with a left-knee injury.
      It would explain some of her unusual statistics against Pliskova, who plays Kerber in Saturday's finale. Knowing she locked down the No. 1 ranking even before her match started against Caroline Wozniacki, the German appeared bolstered and downed Williams' good friend 6-4 6-3.
      "It's just incredible," the Australian Open winner and Olympic silver medalist told the crowd on Arthur Ashe stadium. "To be here in the final for the first time means a lot. And to be No. 1 in the world, it sounds amazing."
      Serving impeccably through the quarters, Williams, meanwhile, slumped to five aces and six double faults. Williams had averaged 12 aces per match in the tournament prior to the last four and struck 18 against Halep.
      Williams, too, committed 31 unforced errors, erring on routine looking forehands and second serve returns.
      "Yeah, I have been having some serious left knee problems," said Williams. "I wasn't tired. Fatigue had absolutely nothing to do with it. If I was tired I should definitely get into a new career.
      "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."
      She suspected the injury surfaced in the second round. The nature of the contest against Halep might have made things considerably worse.
      "I was making errors that I never make, and definitely I didn't make in this tournament in particular," said Williams. "So many simple, simple shots that I easily could have made."
      Williams, however, praised Pliskova, who played a flawless first set and ultimately held her nerve. Pliskova's booming serve shone, particularly in the first set.
      "Karolina played great today," said Williams. "I think if she had played any less then maybe I would have had a chance.
      "So I think I wasn't at 100%, but I also think she played well. She deserved to win today."
      Having ousted Williams' older sister Venus in the fourth round to land in a maiden grand slam quarterfinal -- surviving a match point -- world No. 11 Pliskova became the first player in six years to beat the siblings at the same edition of a tournament. Her winning streak -- which includes a triumph over Kerber last month in the final in Cincinnati -- rose to 11 matches.
      Pliskova relinquished a break advantage midway in the second but prevailed in a tense tiebreak, sealing the affair on Williams' double fault long. Like Williams, she has a sister who plays tennis.
      Pliskova, 24, became the first Czech woman to make the US Open final since 1993.
      She told on-court interviewer Pam Shriver she didn't "believe" what she had accomplished but then added: "Actually I do believe it.
      "I always have a chance to beat anyone if I am playing my game but this is something amazing. I'm really excited to be in the final, especially to beat a player like this.
      "Serena is a champion. It's never easy to play her. Even if she was down a set and a break she was still fighting, so it was very hard. But I was really happy I stayed there and got the win."
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      Pliskova raced out to a 3-0 lead in the tiebreak though Williams won the ensuing three points. The next point felt like a momentum changer.
      With a short forehand and plenty of court space, Williams anticipated where Pliskova would place her forehand and sent her reply to the baseline. The tall Pliskova scurried back to retrieve the ball but lost the point.
      The New York crowd approved.
      But then Williams hit a costly double fault for 4-4. Williams engineered a winning backhand down the line for 5-4, only to lose the final three points. There would be no comeback, the kind witnessed over the years at majors.
      Pliskova had broken for 2-1 in the first and ended the opener by winning 10 of the final 11 points.
      When she secured a further break for 3-2 in the second, Williams was in trouble. Maybe seeing the finish line, Pliskova blinked and was broken back to love.
      They proceeded to hold serve until the tiebreak, when Pliskova completed the upset.
      Williams didn't want to talk about losing the No. 1 ranking but if she wants it back, she will be buoyed by knowing she has no points to defend until the season concludes.
      Kerber became the first German woman to reach No. 1 since Graf, her idol, in 1997. Wozniacki, a former No. 1, was always behind against Kerber, trailing 4-0 in the first and 5-1 in the second.
      So instead of a one-match showdown for No. 1 between Kerber and Williams on Saturday, the former hopes for revenge against Pliskova.