It was tiebreak heartbreak for Serena Williams at the U.S. Open on Thursday night.

Trying to reach her fourth Grand Slam final of the season, trying to win a record-breaking 23rd Slam, Williams instead was ousted from the Open in the semifinals by Karolina Pliskova, 6-2, 7-6 (5), a result that left the Arthur Ashe Stadium crowd stunned.

Pliskova had beaten Serena’s sister Venus in the fourth round, which was a confidence boost for her.

“So I knew it’s gonna be a little bit about the same match what I played with Venus and same serving, same speed,” Pliskova said. “Serena is No. 1. Venus is a little bit behind her. I was expecting tough match, but I knew I can win.”

The 10th-seeded Pliskova had never before made it as far as the fourth round in a Grand Slam. Now she will be playing for a title that would have been Williams’ seventh had she won it all.

Pliskova’s clean, rapier-like groundstrokes and well-placed serves kept Williams on a string all night. Williams, used to dictating points, instead was taking dictation as Pliskova pulled her around the court and kept Williams’ return game to a minimum.

advertisement | advertise on newsday

Pliskova had an easy time in the first set, as if the real Serena had yet to take the court.

In her three-set win over Simona Halep in the quarterfinals, Williams said the real Serena was starting to emerge after a season of reasonable success tempered by the fact that she had won only one (Wimbledon) of the Slam finals she had reached.

She passionately denied that fatigue from the Halep match had anything to do with Thursday night’s loss and said she was having problems with her left knee stemming from an injury earlier in the tournament.

“I have been having some serious left knee problems. I wasn’t tired,” Williams said. “Fatigue had absolutely nothing to do with it. If I was tired, I should definitely get into a new career.”

Williams failed to give herself a break point on Plis kova’s serve in the first set and got to deuce only once. Her own serve, the foundation on which her legendary game was built, just wasn’t there. Pliskova out-aced Williams for the match, 7-5.

@NewsdaySports

Williams was broken in the fifth game of the second set. In the sixth, she finally earned her first break points, though it wasn’t much of her own doing. Pliskova double faulted and committed three errors, giving Williams the break and evening the score at 3-3. The pair held serve to force the tiebreak.

Pliskova won the first three points. Williams won the next four.

The crowd, Williams’ home crowd, was fully behind her. But up 5-4, Williams made a forehand error, a backhand error and a double fault on match point, sending her home and sending Pliskova into the final against Angelique Kerber, who defeated Caroline Wozniacki in the other semifinal.

Williams, who was beaten in the semifinals last year by soft-serving Roberta Vinci, warmly congratulated Pliskova at the net and praised her victory afterward.

“Karolina played great to day,” Williams said. “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 today.”

advertisement | advertise on newsday

And Pliskova finally is realizing her dreams.

“It was always a dream to get a title, get to the semifinal, get to the final,’’ she said. “I hope I didn’t stop yet, that there is still one more step to go. I’ll do anything for getting the title.”