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Roberta Vinci's victory was her first against Serena Williams in five attempts.Credit Chang W. Lee/The New York Times

The Grand Slam will have to wait.

Two victories from joining the most prestigious club in tennis, Serena Williams ran into a roadblock at the United States Open.

In one of the biggest surprises in tennis history, Roberta Vinci, an unseeded Italian veteran playing in her first Grand Slam semifinal, defeated Williams, 2-6, 6-4, 6-4, on Friday.

“It’s an incredible moment for me,” said Vinci, who cried in her chair after the upset and called the victory the “best moment of my life.”

Instead of Williams playing for a Grand Slam in Saturday’s final, it will be an all-Italian final between Vinci and the 26^th-seeded Flavia Pennetta.

In another upset earlier in the day, the 26th-seeded Pennetta defeated No. 2 Simona Halep, 6-1, 6-3. But that, in relative terms, was no surprise compared to what Vinci engineered later in the afternoon on Arthur Ashe Stadium.

Williams came into Friday with a 33-match winning streak at Grand Slam tournaments and had not lost at the U.S. Open since the 2011 final.

Pennetta, 33, and Vinci, 32, have each won Grand Slam titles in doubles, but neither had advanced to a Grand Slam singles final until Friday.

Vinci, ranked 43rd, apologized to the crowd for changing the story line.

“For the American people, for Serena, for the Grand Slam and everything,” she said. “But today is my day. Sorry, guys.”

The last player to complete the Grand Slam by winning all four major singles titles in the same year was Steffi Graf in 1988. Williams had put herself within close range by making great escape after great escape this season, winning the Australian Open, French Open and Wimbledon and her first five matches in Flushing Meadows.

Along the way, she had won 11 three-set matches, shrugging and snarling off early and late challenges, illnesses and other setbacks.

But chasing the Grand Slam is one of the great pressure cookers in sports. Graf has said she felt much more relief than joy when she pulled it off at age 19.

Williams, at 33, was experienced enough to understand just how precious an opportunity this run provided her. Though she had her moments of brilliance on Friday, including 16 aces and many overpowering return winners, the pressure ultimately proved too much.

Down the stretch, she was clearly fighting herself as much as Vinci, a clever tactician whose crisply sliced one-handed backhand made it difficult at times for Williams to generate her trademark pace.

But Vinci is the same player who had never won a set against Williams in their four previous matches, never even pushed her into a tiebreaker.

In Toronto last month, Williams defeated her, 6-4, 6-3, in the quarterfinals. But that will not be the match between Williams and Vinci that will live on in tennis memory.

“How disappointed are you?” was the first question at Williams’s post-match news conference.

“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.”

She then complimented Vinci, saying, “I think she played literally out of her mind.” Williams rejected the idea that she was under a lot of pressure.

“No, I told you guys I don’t feel pressure,” Williams said. “I never felt pressure. I don’t know. I never felt that pressure to win here. I said that from the beginning.”

Williams had won four Grand Slam singles titles in a row, dating back to last year’s United States Open. But the true Grand Slam requires a calendar-year sweep.

Only three women have managed it: Maureen Connolly in 1953, Margaret Court in 1970 and Graf, the swift and serious German aptly nicknamed “Fräulein Forehand” by the American tennis writer Bud Collins. The only men to complete the Grand Slam are Don Budge in 1938 and Rod Laver in 1962 and 1969.

But Williams now joins the short list tennis luminaries who faltered on the final leg: Jack Crawford in 1933, Lew Hoad in 1956 and Martina Navratilova in 1984.

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

Williams did win more points in the match: 93 to 85. But she did not secure those that mattered most in the third set, which had been her happy hunting ground all year and not just in Grand Slam tournaments. Her overall record in three-set matches was 18-1, the only previous loss coming against Belinda Bencic in the semifinals in Toronto last month.

On Friday, Williams smashed her racket in frustration as she sat down in her chair after losing the second set, receiving a code violation for racket abuse from the chair umpire. But when Williams jumped out to a quick 2-0 lead in the third set, it looked as if this would be risky business as usual.

Not this time. She had a game point on her serve for 3-0 at 40-30, but Vinci fought it off with a forehand passing shot winner. Williams spread her arms wide and tried to reassure herself (“It’s O.K.,” she said). But her face was reflecting much more angst than self-assurance.

Vinci got to break point after blocking back a good first serve and getting a backhand unforced error from Williams, who then double faulted to lose her serve and give Vinci a timely boost.

Williams had two more double faults in the seventh game, which she eventually lost with an unforced forehand error.

Vinci now led by 4-3 and a service break. In the next game, she fought off two break points, both of which ended with Williams unforced errors, to go up by 5-3.

Williams cut the lead to 5-4, which gave Vinci plenty of time to think as she sat in her chair before trotting out to serve out the match of her life.

A Williams backhand return in the net: 15-0.

A Vinci backhand half-volley winner: 30-0.

A Williams backhand volley error in the net: 40-0.

Then came match point: a second serve, which could have been a good omen for the aggressive Williams but was not. Vinci parried her big return and soon slapped a forehand approach shot inside out and boldly rushed forward.

Williams’s passing shot landed at Vinci’s feet, and the Italian finished off the ambush in style with a forehand half volley winner into the open court.

The Grand Slam will have to wait, but the first all-Italian women’s final in Grand Slam history will not.