NEW YORK -- Two match wins from tennis history, Serena Williams finally failed in a major moment Friday.
Unseeded Italian Roberta Vinci stunned the No. 1-ranked player 2-6, 6-4, 6-4 in the US Open semifinals for one of the greatest upsets the sport has ever seen. It was Williams' first loss in a Grand Slam singles match this year and ended her bid for the first calendar-year Slam since 1988.
Vinci ended Williams' 2015 major streak at 26 straight matches and, going back to last year's US Open, 33 overall.
Afterward, Vinci called it the best moment of her life.
"It's amazing, like a dream," an emotional Vinci said in her on-court interview. "I'm in the final. I beat Serena. For me, it's an amazing moment, incredible, yes.
"For the American people, for Serena, for the Grand Slam, today's my day. Sorry, guys."
The final stroke, a deft drop shot, brought tears to Vinci's eyes. A 12-1 underdog at the Westgate Las Vegas SuperBook, Vinci admitted that even she did not expect to win. This is the first time a player ranked outside the top 30 has beaten the world No. 1 in the quarterfinals or later at a Grand Slam.
It was also the first time in her career that Vinci had taken so much as a set from Williams -- and now she has reached a Grand Slam final for the first time.
"I thought she played the best tennis in her career," Williams said. "You know, she's 33, and you know, she's going for it at a late age. 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 played literally out of her mind."
Vinci will play Flavia Pennetta, another Italian, in Saturday's final (3 p.m. ET, ESPN). Earlier Friday, Pennetta defeated No. 2 seed Simona Halep 6-1, 6-3.
It's the first time in the Open era that two Italian women will meet in a major final. Pennetta defeated Vinci 6-4, 6-1 in their most recent meeting in the 2013 US Open quarterfinals.
Williams lost a three-set match for only the second time this year, her record now 18-2. After the match, she said pressure did not play a role in the loss.
"I told you guys I don't feel pressure. I never felt pressure," Williams said. "I don't know. I never felt that pressure to win here. I said that from the beginning."
But Williams' mother Oracene Price told ESPN's Jane McManus that the pressure of Grand Slam expectations got to her daughter today. Patrick Mouratoglou, Williams' coach, added that Serena didn't appear to be feeling like herself when he saw her.
The match began shakily for Williams. Vinci, taking advantage of two forehand errors, broke in the first set's third game to take a 2-1 lead. Williams won the next five games to take the first frame in 31 minutes.
There was another brief letdown when Vinci won the first three points of the second set on Williams' serve. But Williams won five points in a row for the hold.
Williams spotted Vinci three more break opportunities in the fifth game, saving the first two with aces. But at 30-40, Williams' backhand strayed wide and Vinci found herself up a break that she never gave back.
Williams settled down early in the third set -- as she has all year. She broke Vinci to take a 2-0 lead but was broken on her subsequent service game.
In a spectacular exchange, Williams had a game point at 3-all but lost it to Vinci, who made a winning forehand volley. Vinci pointed to herself, urging the full house at Arthur Ashe Stadium to root for her.
A tentative backhand into the net and scorched forehand long gave Vinci the break and a 4-3 lead. She saved two break points in the next game to build a 5-3 lead and closed out the match on her next service game.
Williams' record at the US Open against unseeded players is now 44-2; the other loss came to Kim Clijsters in the 2009 semifinals.
Pennetta, having the tournament of her life, sent Halep home in a scant 59 minutes. Trailing 3-1 in the second set, Pennetta won 15 straight points. She closed the match with a flourish, winning 19 of the last 21 points, and is into her first Grand Slam singles final.
Her path to the final has been impressive. Pennetta defeated 2011 US Open champion Samantha Stosur in the fourth round then took out two-time Wimbledon champion Petra Kvitova, the No. 5 seed, in the quarterfinals.