NEW YORK (CBSNewYork/AP) -- For Serena Williams' first 26 matches at major tournaments in 2015, no deficit was too daunting, no opponent too troublesome, no victory too far from reach.
She was unbeaten and, seemingly, unbeatable, nearing the first Grand Slam in more than a quarter-century. All Williams needed was two more wins to pull off that rare feat. And yet, against an unseeded and unheralded opponent in the US Open semifinals, she faltered. Her pursuit of history ended, oh so close.
In one of the most significant upsets in the history of tennis, Williams finally found a hole too big to climb out of, losing 2-6, 6-4, 6-4 Friday at Flushing Meadows to 43rd-ranked Roberta Vinci of Italy.
"I don't want to talk about how disappointing it is for me," Williams said at the start of a briefer-than-usual news conference. "If you have any other questions, I'm open for that."
GALLERY: Serena Stunned In US Open Semifinals
Vinci had never before played in a Grand Slam semifinal; Williams owns 21 major titles. In four previous matchups, Vinci had never taken a set off Williams.
"Every so often," Vinci said, "a miracle happens."
How little faith did even she have? Vinci said she booked a flight home for Saturday, the day of the final.
But Vinci's unusual style, full of slices and net rushes, kept Williams off-balance enough to cause problems and prevent the 33-year-old American from becoming the first player since Steffi Graf in 1988 to win all four Grand Slam tournaments in a calendar year.
As Williams quickly left the scene, hopping in a waiting black SUV and taking off, her coach, Patrick Mouratoglou, was explaining to reporters what he called "a bad day, clearly."
He said he could tell before the match that something was off.
"She was very slow. There was no movement with her lower body, so she was in bad positions to be aggressive and play her attacking game," Mouratoglou said. "She couldn't find it today. You don't wake up the same way every day. Some days you feel good, other days you don't feel good. That's life. Usually she finds a way, and today she did not."
Williams had been pushed to the limit before — this was her 12th three-setter in a major this season — but had managed to win titles at the Australian Open on hard courts in January, the French Open on clay courts in June, and Wimbledon on grass courts in July. And she had won five matches on the US Open's hard courts over the past two weeks.
This time, for once, the No. 1-ranked Williams could not pull it out, undone by 40 unforced errors, twice as many as Vinci. That negated the impact of Williams' 16 aces, including one at 126 mph.
"I thought she played the best tennis in her career," Williams said about Vinci. "She played, literally, out of her mind."
Vinci next faces another Italian making her Grand Slam final debut: 26th-seeded Flavia Pennetta, who eliminated No. 2 Simona Halep 6-1, 6-3 in another, if less-unbelievable, surprise.
Pennetta, 33, and Vinci, 32, have known each other since there were kids, growing up in towns about 40 miles (65 kilometers) apart on opposite coasts of Puglia, a region in the southeastern heel of Italy's boot-shaped peninsula. They used to meet in local tournaments in their early teens, then paired up to win a French Open junior doubles in their late teens.
Now, all these years later, they will face each other in a stadium in New York with a Grand Slam trophy on the line.
"We'll be as tight as violin strings, both of us," Vinci said.
She gave a thumb's up while noting in English that "an Italian wins, for sure," then pointed to her chest and whispered in Italian, "Me, let's hope."
An intriguing story line, to be sure, but nothing compared to what Williams was chasing: a perfect Grand Slam season.
After all, not only was she 26-0 at those tournaments this year, but her winning streak at majors was 33 matches, because she won last year's US Open. If she had managed to win a fifth consecutive major title, Williams would have raised her total to 22, equaling Graf for the most in the Open era, which began in 1968, and second-most in history behind Margaret Court's 24.
"I never felt that pressure to win here," Williams insisted. "I said that from the beginning."
Her older sister Venus — who pushed her to three sets in the quarterfinals — was in her guest box, and rapper Drake, a pal, was in Arthur Ashe Stadium, too, Friday.
Williams grabbed six games in a row to take the first set and go ahead 1-0 in the second. Suddenly, though, Vinci broke to go up 3-2 in the second. When Vinci served out that set, Williams headed to the sideline, cracked her racket against the ground and flung it behind her chair, drawing a code violation warning from the chair umpire.
"I saw she was nervous," Vinci said, "and that helped me."
As the third set wore on, Williams became more and more demonstrative, leaning forward, shaking her fists and screaming, "Come on!" after four points in a row during one stretch. Vinci showed emotion later, closing a spectacular 18-stroke exchange with a volley winner, then cupping her ear with a hand, before pointing to her chest and waving both arms at the spectators, encouraging them to yell for her, too.
That point ended with Williams on a full sprint, stumbling a bit as her momentum carried her to the sideline, where she bent over, chest heaving.
The crowd responded to it all with a standing ovation.
Williams never was able to get back in front. She let a 2-0 lead in the third set slip away, in part by double-faulting on break point to make it 2-1. Williams double-faulted again a few games later, when Vinci broke for 4-3, a lead she did not relinquish.
"I mean, I made a couple of tight shots, to be honest," Williams acknowledged, "but maybe just about two."
If that's truly all it was, that's all it took.
(TM and © Copyright 2015 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2015 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)
for more features.