WHAT HAPPENED: Karolina Pliskova crashed the US Open final weekend party, toppling world No. 1 Serena Williams, 6-2, 7-6, in Thursday night’s semifinals to silence a partisan Arthur Ashe stadium crowd.

“It’s amazing to beat a player like this, “said Pliskova. “I don’t believe it.”  

This has been a breakthrough Grand Slam for the lanky 10th-seeded Czech, who now awaits the winner of the late semifinal between second-seeded Angelique Kerber and unseeded Caroline Wozniacki in what will be her first major final on Saturday.

“I don’t care who is in the final," said the 24-year-old, the first Czech to reach the women's singles final since Helena Sukova lost in 1993 to Steffi Graf.

It is the second stunning consecutive semifinal exit for Williams, and the second with history on the line. Last year, she needed just two more wins to complete a year Grand Slam before being surprised by unseeded Roberta Vinci. 

This time, again, Williams lost more than just a semifinal. With the defeat, she will yield the world No. 1 ranking to Kerber after a 186-week reign. At this year's Open, she was also bidding for her 23rd career Grand Slam singles title, which would haven broken a tie with Steffi Graf for most in the Open era, and her seventh US Open singles crown, which would have moved her ahead of Chris Evert for the most in the Open era and into tie with Helen Wills for second on the all-time list, one behind the eight titles Molla Bjurstedt Mallory win in the early 1900s.  

This match was a quick turnaround for the 34-year-old Williams after fighting through a two-hour, 14-minute slugfest Wednesday night against No. 5 Simona Halep. Afterward, however, Williams said conditioning was not a factor.

“Fatigue had nothing to do with,” said Williams. “Being a professional, if I am not able to recover, I am not going to make excuses.”

However, Williams mentioned that she had a “serious left knee injury” suffered earlier in the tournament that slowed her down and distracted her throughout the one-hour-and-25-minute match.

“I was not 100 percent,” said Williams. “My mind was elsewhere, you are thinking of other things. I was making errors on simple and simple shots, just blame that on thinking on my leg."

The first and last points of the match captured the American’s uneven play. Williams opened the match with an ace. She finished the match with a double fault, her sixth of the match, giving Pliskova the tiebreak, 7-5, and the win.

Williams had led 4-3 in the breaker after scoring the point of the match, when she lofted a desperate backhand lob on the line and followed that with a winner. But she gave back the advantage by double faulting the next point.

Williams went up again, 5-4, but then lost her form, missing two ground strokes and then double faulting on match point.  

In the opening set, Pliskova broke Williams in the third game and again in the seventh game for a 5-2 lead. She served out the set with a love game.

WHAT IT MEANS: While unexpected, Pliskova has been on a roll this summer, winning 22 of her last 26 matches and a tournament title in Cincinnati. Pliskova will rise to a career-high ranking of at least No. 6 after tournament, eclipsing her previous best of No. 7 last summer.  Having beaten Venus Williams in the fourth round, Pliskova is the first player since Kim Clisters in 2009 to beat both Williams sisters in the same Open, and just the third overall. Justine Henin also accomplished the feat in 2007.

THE QUESTION: After such a huge win, can Pliskova fight off a letdown and regroup mentally to be sharp in the final?