NEW YORK – On the cusp of history, Serena Williams couldn't write the final chapter Friday.

With a calendar year Grand Slam on the line, Williams lost in the U.S. Open semifinals to unseeded Roberta Vinci, who had never beaten Williams in four previous matches, had never won a set from her or taken more than four games in one.

Then Friday, the 32-year-old Italian used her considerable guile to upset the No. 1 player in the world in a result that no one could have foretold, just as it couldn't be predicted that she would be facing Flavia Pennetta, her 33-year-old countrywoman, in Saturday's final. Vinci won 2-6, 6-4, 6-4, and Pennetta beat the No. 2 player in the world, Simona Halep, with surprising ease, 6-1, 6-3.

The Arthur Ashe Stadium crowd was stunned.

Shortly after the match, Williams credited Vinci for the quality of her play and dismissed the question of disappointment for not adding the Open title to her Grand Slam portfolio this season of the Australian Open, French Open and Wimbledon.

"I don't want to talk about how disappointing it is for me," Williams said. "If you have any other questions, I'm open for that."

Unseeded Roberta Vinci’s upset made her a most unlikely half of an all-Italy final at the Open.

Williams rallied to take the first set after being broken in the third game. Then Vinci craftily worked her way into the match, starting to serve better, moving Williams from side to side and from backcourt to the service box, where she seemed uncomfortable.

"She played the best tennis of her career. She's 33 [actually 32], she's going for it at a late age," said Williams, who turns 34 this month. "So that's good for her to keep going for it and playing so well. Actually, I guess it's inspiring. But yeah, I think she literally played out of her mind."

Though Williams served 16 aces and only four double faults, her power didn't seem to phase Vinci, who picked up her timing on the serve as the match progressed.

"It's amazing. It's like a dream," Vinci said. "I'm really happy, but of course I'm a little bit really sad for Serena because she's incredible player, No. 1."

Toward the end of the deciding set, Vinci had to harness her emotions. "When I made the break [for 4-3] and serve, I was a little bit scared," she said. "My arms were like [she demonstrated shaking]. I'm not joking. Stay calm, relax, breathe every single point. Don't think you have Serena on the other side of the court."

For her part, Williams said she was satisfied with the way she played.

"I don't think I played that bad," Williams said. "I made more unforced errors than I normally would make [34 groundstroke errors], but I think she played really well. She did not want to lose today. Neither did I, incidentally."

• Roger Federer and Novak Djokovic rolled to straight-sets victories in the men's semifinals. Federer, who has not dropped a set, beat Stan Wawrinka 6-4, 6-3, 6-1, while the top-ranked Djokovic ousted defending champion Marin Cilic 6-0, 6-1, 6-2 in 85 minutes.