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Serena Williams upset by Karolina Pliskova in US Open semifinals, will lose No. 1 ranking

For the second straight year, Serena Williams was upset in the US Open semifinals.

Whereas the American was defeated by a crafty veteran in Roberta Vinci 12 months ago, a struggling Williams lost to big-hitting Karolina Pliskova 6-2 7-6 (7-5) on Thursday in New York.

She’ll thus have to wait to become the first player in the Open Era to win 23 majors — Williams is currently tied with Steffi Graf on 22. And in a double blow, when the new rankings are released Monday, the 34-year-old will lose top spot to Angelique Kerber, ending Williams’ record-tying — with Graf — 186-week stay at No. 1.

Having ousted Williams’ older sister Venus in the fourth round — surviving a match point — world No. 11 Pliskova became the first player in six years to beat the siblings at the same edition of a tournament. Her winning streak rose to 11 matches.

She will meet either Kerber or Caroline Wozniacki in the final.

Pliskova, 24, played a flawless first set, helped by her booming first serve. She relinquished a break advantage midway in the second but prevailed in a tense tiebreak, sealing the affair on Williams’ double fault long. Like Williams, she has a sister who plays tennis.

Perhaps Wednesday’s exertions — she played a grueling encounter against Simona Halep — took a toll on Williams. Serving impeccably through the quarters, Williams hit five aces and six double faults while amassing 31 unforced errors against Pliskova.

Pliskova became the first Czech woman to make the US Open final since 1993.