Serena Williams of the United States reacts to a point against Karolina Pliskova of the Czech Republic on day eleven of the 2016 US Open tennis tournament at USTA Billie Jean King National Tennis Center in New York, the United States, Sept 8, 2016. [Photo/Agencies]

NEW YORK - The 24-year-old Czech Karolina Pliskova upset No 1 seeded Serena Williams in the US Open semifinals 6-2, 7-6 (5) on Thursday.

"I made a lot of errors and I didn't play as well as I have been playing," Williams said, "She (Pliskova) was a good, all-around player."

Williams had won her 22nd major at Wimbledon in July, tying Steffi Graf's Open era record with the win. But tonight's loss means she will not get to 23 majors this US Open.

"I don't believe it. Serena is a champion. It's never easy to play her. I'm really happy that I stayed there to get the win," said Pliskova.

The 10th-seeded Czech player, who had never made it past the third round of a major before, makes her first Grand Slam final.

Pliskova also became the fourth woman to defeat both Williams sisters in the same Grand Slam with a 4-6, 6-4, 7-6 (3) win over Venus Williams in the fourth round.