New York
Williams, who clutched at her left leg between points in the second set, double-faulted to end it.
Afterward, her coach, Patrick Mouratoglou, said Williams injured her left knee in the quarterfinals.
The loss prevents Williams from earning her seventh championship at Flushing Meadows and 23rd major title overall, which would both have been Open-era records.
It also means Williams’ 3½-year reign at No. 1 in the WTA rankings will end. She will be overtaken on Monday by current No. 2 Angelique Kerber, defeated Caroline Wozniacki in Thursday’s 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 in the semifinals.
This was the 33rd major semifinal of Williams’ career, and the first for Pliskova, who beat the 34-year-old American’s older sister Venus in the fourth round. Pliskova is only the fourth woman to beat both Williams siblings during the same Grand Slam tournament.
And to think: The 24-year-old Pliskova had never been past the third round in 17 previous appearances at majors.
But on Thursday, she certainly looked the part of an up-and-comer with the strokes and demeanor to go far.
The temperature was in the low 80s, and the air was muggy and still, and Williams kept using the pleats of her black-and-pink skirt to wipe her sweaty palms between points.
Watching Williams miss shot after shot — 31 unforced errors in all — one couldn’t help but wonder why.
One thought: Maybe it was a recurrence of the soreness in her right shoulder that became bothersome in the days right after she won singles and doubles titles at Wimbledon two months ago. Or perhaps it was the toll of the grueling three-set quarterfinal against Simona Halep that concluded less than 22 hours before the semifinal started. But by the latter stages, Williams kept reaching for her left leg.
“She was not moving at all today,” 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 the American’s own game presents others.
Kerber reached her third major final this year after dispatching Caroline in straight sets.
Kerber earned her first career No. 1 ranking before she even stepped on court Thursday with Serena Williams’ loss in the first semifinal. Then Kerber went out and looked like a top-ranked player, winning the first four games en route to a 6-4, 6-3 victory.
The German lefty will face Pliskova on Saturday in a rematch of the final at Cincinnati nearly three weeks earlier. Kerber would have taken over the No. 1 ranking then had she won that day, but Pliskova instead captured a breakthrough title and has ridden that momentum since.
Kerber had never made a Grand Slam final at the start of this year. Then she not only advanced that far but won the title at the Australian Open, beating Williams in the championship match. She followed that up with a run to the Wimbledon final, where she lost to Williams.
Wozniacki is a two-time runner-up at Flushing Meadows, but she arrived at this year’s tournament at No. 74 in the world after missing 2½ months because of an ankle injury. She was seeking to become the lowest-ranked women’s finalist at the U.S. Open, other than an unranked Kim Clijsters in 2009.