Roberta Vinci of Italy celebrates defeating Serena Williams in their semifinal match Friday at the U.S. Open. (Mike Segar/Reuters)

Roberta Vinci worked No. 1 Serena Williams back and forth across the baseline during an 18-shot rally midway through a third set Friday that few saw coming in the U.S. Open semifinals.

Vinci, playing in her first career Grand Slam semifinal, finished the point by attacking the net and hitting a backhand volley winner. The 32-year-old Italian then cupped her ear with her hand, raised her arms to the crowd and pointed at her chest. She was there to play, too.

And she was there to win as well, capping a stunning 2-6, 6-4, 6-4 victory with a forehand winner that ended Williams’s quest to become the first woman to win all four major tennis titles in the same calendar year since Steffi Graf in 1988.

As the magnitude of her victory sank in, Vinci lifted her arms to the crowd then bent over and covered her face with her hands.

“This is an incredible moment for me,” Vinci, who is ranked No. 43 in the world, said in an on-court interview after the match. “It’s amazing, like a dream. I’m in the final, I beat Serena, I’m — phewwww. . . .”

“Sorry guys,” Vinci continued, looking to the crowd. “Sorry, but for me it’s an amazing moment. It’s incredible.”

Two unforced errors by Williams gave Vinci a 4-3 lead in the final set before she got her ankle retaped on the changeover. Williams had dropped a set 11 times in Grand Slam events this year, but Friday’s brink seemed as inconvertible as any she has faced in her pursuit of the her fourth major title of the year.

The two held serve before Vinci had the opportunity to serve for the match, up 5-4.

Williams hit Vinci’s serve into the net on the first point of the game, and the Italian attacked the net to hit a winner on the next point to move two points away from ending Williams’s run. An error from Williams on the next point gave Vinci triple match point at 40-0 as the match hit the two-hour mark.

“I don’t want to talk about how disappointing it is for me, if you have any other questions,” Williams said at her post-match news conference. “I thought she played the best tennis of her career. . . . She played literally out of her mind.”

Williams, who breezed by Vinci a month ago in Toronto and held a 4-0 head-to-head edge, took the first set of Friday’s semifinal despite losing a service game early.

The world No. 1 finished that set by winning 24 of 32 points as her sister, Venus Williams, and a boisterous Arthur Ashe crowd looked on.

Williams could not come back from a Vinci break in the second set, though, failing to capitalize on two break-point opportunities.

“I don’t think I played that bad,” Williams said. “I made more unforced errors than I normally would, but I think she just played really well. She did not want to lose today, and neither did I, incidentally, but she really didn’t either.”

Williams’s last loss at the U.S. Open was in the 2011 final. She won the tournament the next three years.

Vinci and Williams were originally scheduled to play Thursday night, but their match was moved to midday Friday because of rain.

Williams is 25-4 in Grand Slam singles semifinals and 46-8 in the final two rounds at majors.

In the final, Vinci will face fellow Italian Flavia Pennetta, who upset No. 2 Simona Halep on Friday to reach her first career Grand Slam final at 33 years old.

Pennetta, ranked No. 26, has a 5-4 lifetime advantage over her countrywoman.