NEW YORK >> Serena Williams was upset in the U.S. Open semifinals for the second year in a row, beaten 6-2, 7-6 (5) by 10th-seeded Karolina Pliskova of the Czech Republic on Thursday night.
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' 31/2-year reign at No. 1 in the WTA rankings will end. She will be overtaken on Monday by current No. 2 Angelique Kerber, who was to face 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.
Pliskova serves well, leading the tour in aces this season, and her angled offerings gave Williams fits. She also returns well, often sending stinging responses right at Williams' feet, leaving her no time to react properly. And when they engaged in lengthy baseline exchanges, Pliskova's deep, flat groundstrokes were able to produce the rare sight of Williams back on her heels.
All of 7 minutes in, Pliskova showed she was not shaken by any of it — the stage, the setting, the stakes or the foe — and was going to make this a struggle for Williams. Getting all sorts of pace on returns, including on a 104 mph serve at break point, when Williams sent a backhand long to trail 2-1.
Williams yelled out, "Ai-yai-yai!" and slapped her left thigh.
Pliskova broke again, this time at love, to lead 5-2 thanks to a double-fault and a trio of miscues by Williams. That was part of a stretch in which Pliskova grabbed 10 of the first set's last 11 points.
Not much later, things were looking decidedly dicey for Williams when she sailed a backhand long to get broken and trail 3-2 in the second. Pliskova, who betrayed little emotion otherwise on this evening, leaned over and pumped her fists.
They had played for 46 minutes, and it was clear who was better. Pliskova sat in her changeover seat, leaned back and exhaled. And then, for the very first time, she showed some nerves, coming out and playing her shakiest game of the match to get broken at love.
Into the tiebreaker they went, and Pliskova jumped ahead 3-0. Then came Williams, never one to back down, going ahead 5-4, just two points from forcing a third set. But she wouldn't take another point, closing the surprising loss with a sixth double-fault and leaving Flushing Meadows with another disappointment.