NEW YORK • The semi-final stage of the US Open was the departure point for Serena Williams last year, a surprisingly early exit in her hunt for the Grand Slam.
Perhaps the pressure of trying to win all four Major tournaments in one year was the culprit, or perhaps it was merely the unexpected brilliance of Roberta Vinci that day.
One year on, there was less pressure on Williams, but even more brilliance from her opponent.
Tenth seed Karolina Pliskova, playing in a Grand Slam semi-final for the first time, showed no nerves and none of her inexperience in defeating Williams 6-2, 7-6 (7-5) in a shock result on Thursday at the Arthur Ashe Stadium.
By summoning some incredible poise to handle the pressure, she became the first Czech woman to reach the US Open final since Helena Sukova in 1993.
"I was prepared for anything," Pliskova said. "Even if I would have lost the second set, I would be ready for the third one."
Williams, who played a tough three-set match the previous night against Simona Halep, refused to blame fatigue. But she did say that a left-knee injury that cropped up the night before made a difference.
"Yeah, I've been having some serious left-knee problems," Williams said, adding: "I wasn't able to move the way I wanted to.
"When you're injured, you're thinking of other things when you should be just playing and thinking of your shots. My mind was just a little bit everywhere."
The loss was a profound one for Williams, who played erratically in producing 31 unforced errors against only 20 winners.
It prevented her from reaching several milestones, and knocked her off her long-held throne as the No. 1 player in the world.
Angelique Kerber, who beat Caroline Wozniacki 6-4, 6-3 in the other semi-final, will officially take over the No. 1 ranking on Monday.
Pliskova and Kerber will meet in the women's final today.
Had Williams won the Open, she would have surpassed Steffi Graf's record of 186 consecutive weeks at No. 1. Instead, they will remain tied.
The American was also seeking her 23rd Grand Slam tournament win, which would have broken a tie with Graf for the most titles in the Open era.
Williams also failed to break a tie with Chris Evert for the most US Open championships. They each have six.
NEW YORK TIMES, AGENCE FRANCE-PRESSE