NEW YORK — Serena Williams was upset in the U.S. Open semifinals for the second year in a row, beaten 6-2, 7-6 (7-5) by 10th-seeded Karolina Pliskova of the Czech Republic on Thursday.
Williams, who clutched her left leg between points in the second set, double-faulted to end it. Williams confirmed in her postmatch interview that she had been dealing with an injured left knee since the second or third round and it curtailed her movement. She was adamant that fatigue was not a factor, having gone three sets against Simona Halep in the quarterfinals on Wednesday night.
"I definitely wasn't tired," she said. "It wasn't a five-hour match.
"Karolina played great today. I think if she had played any less, then maybe I would have had a chance. So I think I wasn't at 100 percent, but I also think she played well. She deserved to win."
The loss prevents Williams, 34, from earning her seventh championship at Flushing Meadows and 23rd major title overall, which would both have been Open era (from 1968) records.
It also means Williams' 3½-year reign at No. 1 in the WTA rankings ends. She will be overtaken Monday by No. 2 Angelique Kerber, who advanced to the final by beating Caroline Wozniacki 6-4, 6-3 in the second semifinal.
A year ago, Williams' bid for a calendar-year Grand Slam ended when she lost in the U.S. Open semifinals to unseeded Roberta Vinci of Italy.
It was the 33rd major semifinal of Williams' career and the first for Pliskova, who beat Williams' sister Venus in the fourth round. Pliskova is the fourth woman to beat both Williams siblings during the same Grand Slam tournament.
"Obviously, the match with Venus helped me … not only with the game but also with the crowd," Pliskova said. "Was my first match on center court."
And to think: Pliskova, 24, had never been past the third round in 17 previous appearances at majors. But Thursday, she looked the part of an up-and-comer with the strokes and demeanor to go far.
But watching Williams miss shot after shot — 31 unforced errors in all — one couldn't help but wonder why.
"She was not moving at all today," Williams' coach, Patrick Mouratoglou, said. "There was no match."
Still, Pliskova surely had a lot to do with Williams' woes. Pliskova's power is of the sort that Williams so rarely is forced to confront, much like the difficulties Williams' game presents others.
Pliskova faces Kerber on Saturday in a rematch of the final at Cincinnati nearly three weeks earlier. Kerber would have taken over the No. 1 ranking had she won that day, but Pliskova captured a breakthrough title and has ridden that momentum since.
Kerber had never made a major final at the start of this year. Then she won the Australian Open title, beating Williams in the title match. She followed that up with a run to the Wimbledon final, where she lost to Williams.