Serena Williams stunned by Karolina Pliskova in U.S. Open semifinals

NEW YORK — Was it nerves? A bad left knee? An opponent playing her best in the biggest match of her career?

For Serena Williams, it could have been any of those reasons — or all — but none of those changed the fact that Thursday night at the U.S Open, the world No. 1 lost for the second straight year here in the semifinals, just two matches from history.

“I don’t think much went well today,” a subdued Williams told reporters after a 6-2, 7-6(5) shocker at the hands of Karolina Pliskova, a first-time major semifinalist. “I made a lot of errors. I didn’t play as well as I’ve been playing.”

A bad left knee visibly hampered her late in the second set. Coach Patrick Mouratoglou said they had worked all day to get it ready. Williams said it was on her mind throughout the 85-minute defeat.

“I don’t remember having seen her move so slowly ever,” Mouratoglou said. “As soon as she started, it was terrible.”

While Pliskova, a 24-year-old from the Czech Republic, advances to her maiden major final, Williams is stopped short of winning a record 23rd Grand Slam.

She is halted there for now, tied with Steffi Graf for the record of Open era Slams at 22. She loses her world No. 1 ranking, as well, gone from the top for the first time in 186 weeks — another record she now must share with Graf.

With her mind on the injury, Williams' game went awry. She hit 31 unforced errors in the match, broken three times and double faulting six, including on match point down.

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“I don’t blame anything today,” Williams said, going on to explain that her injury did not have anything to do with the fact that she played on back-to-back nights for the first time this U.S. Open.

At one point she got stern in pointing that out: “I'm not going to repeat myself. I wasn't tired from yesterday's match.”

Pliskova, for her part, held her nerve. She blasted seven aces and stood firm when Williams rallied in the second set tiebreak with some sensational shot making. Having never advanced to the second week of a major prior, Pliskova now becomes just the fourth woman to have beaten both Venus and Serena at the same major, and first to do so since Kim Clijsters at the 2009 U.S. Open.

Pliskova will meet Angelique Kerber in Saturday's final, Kerber beating a resurgent Caroline Wozniacki in the night's second semifinal, 6-4, 6-3.

Kerber, 28, becomes the oldest player to reach No. 1 for the first time when the new rankings come out Monday.

Williams, shrugging off questions about the ranking, said she wasn’t sure what her schedule was for the rest of the year.

Her coach said the disappointment will be palpable, however.

“Not winning (this) title will bother her much more, but sure, she’s not going to be happy not to be No. 1 anymore,” Mouratoglou said. “It’s winning or nothing for her. Final or first round, it doesn’t matter. She didn’t win the tournament. At this point in her career, she cannot be happy just being in the final.”

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