NEW YORK >> For the second year in a row, Serena Williams’ bid to make history ended with a shocking loss in the U.S. Open semifinals.

A seventh title at Flushing Meadows, which would have been an Open-era record, will have to wait. So will a 23rd Grand Slam championship, another record. And her 3½-year reign at No. 1 in the WTA rankings is over, too, one week shy of what would have been yet another mark.

Undone by a half-dozen double-faults, including on match point, and plenty of other mistakes she blamed in part on dealing with an injured left knee, Williams was upset 6-2, 7-6 (5) by big-serving Karolina Pliskova of the Czech Republic on Thursday night.

“Karolina played great today. I think if she had played any less, then maybe I would have had a chance,” Williams said. “So I think I wasn’t at 100 percent, but I also think she played well. She deserved to win today.”

The 10th-seeded Pliskova, who will play No. 2 Angelique Kerber of Germany for the title on Saturday, began her on-court interview by blurting out that she couldn’t believe she’d eliminated Williams to earn a spot in her first major final. Then Pliskova changed course, saying: “I mean, actually, I do believe it. I always knew I have a chance to beat anyone if I’m playing my game.”

At the 2015 U.S. Open, Williams’ bid for a calendar-year Grand Slam ended when she lost in the semifinals to unseeded Roberta Vinci of Italy in one of the biggest surprises in tennis history. This one goes pretty high on the list, too.

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 after staving off a match point. Pliskova is only the fourth woman to beat both Williams siblings during the same Slam.

“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: The 24-year-old Pliskova had never been past the third round in 17 previous appearances at majors.

But now she’s on an 11-match winning streak.

“One more step to go,” she said.

Kerber reached her third Grand Slam final of the season — and of her career — by beating two-time U.S. Open runner-up Caroline Wozniacki 6-4, 6-3. Kerber beat Williams in the Australian Open final in January, then lost to her in the Wimbledon final in July.

Regardless of Saturday’s outcome, Kerber will move up one spot to No. 1 on Monday, ending Williams’ stay of 186 consecutive weeks, exactly the same number Steffi Graf compiled for the record. Kerber had a chance to overtake Williams last month by winning a hard-court tournament in Ohio, but lost the final there to Pliskova.

Williams, who refused to answer questions about the rankings, and Graf also share the mark for most Grand Slam titles since 1968, when professionals were first admitted to majors. And Williams remains even with Chris Evert with six U.S. Open trophies.

Pliskova, meanwhile, is now one victory from her first, and on Thursday night, 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 Williams kept using the pleats of her black-and-pink skirt to wipe her sweaty palms between points. She made 31 unforced errors in all.

Afterward, Williams dismissed the notion she was fatigued from a grueling three-set quarterfinal against Simona Halep that concluded less than 22 hours before the semifinal started (Pliskova, whose quarterfinal was earlier Wednesday, watched, then slept in until 1 p.m. on Thursday).

Instead, Williams and her coach, Patrick Mouratoglou, pointed to the left leg that she repeatedly clutched.

He said she spent far less time than usual warming up for the match, after getting treatment much of the day.

“When you’re hampered, you’re thinking of other things. Like, I was making errors that I never make, and definitely I didn’t make in this tournament in particular. So many simple, simple shots that I easily could have made,” Williams said. “I just blame that on just mentally thinking about my leg and just not thinking about the shot.”

Mouratoglou was more succinct.

“She didn’t show up. She couldn’t play,” he said. “She was so slow. She couldn’t move.”

Pliskova said she was focusing on herself and didn’t notice whether Williams was troubled.

“If someone is not ready, and she doesn’t think she is ready,” Pliskova said, “she should not go on the court.”

Either way, Pliskova surely contributed to 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.

The 6-foot-1 Pliskova serves well, leading the tour in aces this season and averaging 109 mph Thursday, and her angled offerings gave Williams fits. Pliskova returned well, often sending stinging responses right at Williams’ feet, leaving her no time to react properly. And Pliskova’s deep, flat groundstrokes produced the rare sight of Williams 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 broke to lead 2-1. Williams yelled out, “Ai-yai-yai!” and slapped her left thigh.

Pliskova would go on to win 10 of the first set’s last 11 points.

In the second-set tiebreaker, Pliskova led 3-0. Then came Williams, never one to back down, going ahead 5-4, two points from forcing a third set. But she wouldn’t take another point, closing the surprising loss with another double-fault, leaving Flushing Meadows with another so-close-but-so-far disappointment.

“It’s winning or nothing. Final or first round, it’s the same,” Mouratoglou said. “She didn’t win the tournament.”