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 on that sunny day.
A year later, there was less pressure on Williams, but even more brilliance from her opponent.
Tenth-seeded 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 (5), in a shocker on Thursday night in Arthur Ashe Stadium.
By summoning some incredible poise to handle the moment, 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 night before against Simona Halep, refused to blame fatigue. But she did say that an unspecified left knee injury that cropped up the night before made a difference against Pliskova.
“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."
Her coach, Patrick Mouratoglou, said that Williams’ movement was so restricted that she could not reach balls she normally does and could not attack.
“If there had been a third set," he said, “it would have been 6-0."
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.
That distinction goes to Angelique Kerber, who beat Caroline Wozniacki, 6-4, 6-3, in the other semi-final. Pliskova and Kerber will play in the women’s final on Saturday.
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 in that category.
Williams 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.
“I’m a perfectionist, and I love to win," Williams said. “That’s when I feel my proudest."
Pliskova, on the other hand, had never even made it past the third round of a major tournament before this week. Once there, she plowed through the draw like a 10-time champion.
She beat Venus Williams in the fourth round, becoming the eighth player to beat both Williams sisters in the same tournament.
Pliskova said that playing Venus Williams in her first-ever match in Ashe Stadium had prepared her for the experience she faced Thursday night against a crowd that overwhelmingly supported her opponent. Pliskova knew she was not going to win any popularity contests.
“I knew I was to play center court one of the Williams sisters against all the people there," she said. “I was prepared for it. I was prepared for a fast game, for great serving, and probably it helped me. That’s why maybe I won the match today."
A potent server with powerful groundstrokes to match, the 24-year-old Pliskova has been viewed by some as an underachiever so far in her career. But on Thursday she achieved the biggest win of her life.
Williams is generally considered the best server in the women’s game, but Pliskova was better. She had seven aces to Williams’ six and won 84% of the points off her first serve, compared with 66% for Williams.
Williams had fewer double faults (7-6), but the last one came at the worst possible moment: on match point in the second-set tiebreaker.
“I definitely think I could serve better," Williams said. “But that’s the beauty of the sport. Always opportunities to do better."
Pliskova broke Williams’ serve twice in the first set, winning it in only 26 minutes. In the first game of the second set, she demonstrated her nerve by refusing to be intimidated by Williams in a volatile moment at the net.
Williams drove a forehand directly at Pliskova, who got her racket on it in self-defense. The ball deflected wide. Williams made a fist, turned in Pliskova’s direction and shouted, “Come on," with more than her usual ferocity, and the crowd roared in delight.
These are the moments when many players wither and shrink. But Pliskova held firm. Even later, when asked if she was bothered that much of the focus was on Williams’ knee problem, Pliskova refused to let it distract her from happiness.
“If someone is not ready and she doesn’t think she is ready, she should not go on the court," Pliskova said. “I would say it’s still her choice if she wants to play or if she is ready to play or not. I’m just happy with my win, and I’m not going to think about something like this."
For Williams, the Grand Slam season ends with one major title, at Wimbledon. For now, she will have to be content with 22 major championships. Pliskova is hungering for her first.
“I will do anything to win the title here," she said.
She has already accomplished much more than she ever has before. AP