Serena Williams saw her dream of a calendar year Grand Slam title sweep shattered in stunning fashion on Friday as Italy's Roberta Vinci shocked the world number one to reach the US Open final.
Vinci battled back for an epic upset of the three-time defending champion 2-6, 6-4, 6-4, ending Williams' 33-match Grand Slam win streak.
In Saturday's final, world number 43 Vinci will face 26th-seeded compatriot Flavia Pennetta -- who upset Romania's second-ranked Simona Halep 6-1, 6-3 -- in the first all-Italian Grand Slam women's final in the Open Era.
"I think she played out of her mind. I don't think I played that bad. I think she just played really well," said Williams. "She played the best tennis of her career."
It was a shocking end to Williams' run at not only a sweep of the year's four major titles, a feat that hasn't been done since Steffi Graf's 1988 effort, but her bid for a 22nd Grand Slam title to match Graf's Open Era career record and a take a fifth Slam crown in a row.
"I don't want to talk about how disappointing it is for me," said Williams in a brief post-match news conference.
That left it for her coach, Patrick Mouratoglou, to sum up the failure.
"The pressure of this tournament was enormous and this was a match which she managed less well than the others," he said.
"After her career, what we will remember isn't that she didn't win the Grand Slam. We will say that she is the greatest champion of all time."
Vinci was overjoyed by her shock win.
"It's an incredible moment for me," Vinci said.
"It's like a dream. I'm in the final. I beat Serena. It's an amazing moment. It's incredible.
"It's I think the best moment of my life."
Vinci apologized to the crowd for ending Williams' Grand Slam sweep dream.
"Sorry for the American people, for Serena, for the Grand Slam and everything. Today is my day. Sorry guys."
Vinci, 32, said she never thought to win after losing four prior matches against Williams without taking a set.
"No, really, it's true," she said. "When I wake up, I say, 'I have a semi final today. I play Serena. Try to enjoy.' I didn't expect that I win."
Men's world number one Novak Djokovic, who will seek his third Slam title of the year against Roger Federer on Sunday, warned that the defeat might make Williams tougher.
"Certainly isn't easy for Serena to deal with this loss today, but she's a great champion," Djokovic said. "She has been there before. She knows how to take the lessons out of this experience and move on as a stronger player."
Pennetta and Vinci become only the third and fourth Italian women in a Slam final after Francesca Schiavone won the 2010 French Open final and lost in 2011 and Sara Errani lost the 2012 final at Roland Garros.
Pennetta, who took just 59 minutes to dispatch Halep, leads Vinci 5-4 in career meetings.
"It's amazing," Pennetta said. "I didn't think to be so far in the tournament. I played really well. I tried to play every match the best I can from the first one, don't worry about the draw and focus on the things I have to do on the court. And here I am."
Williams, who fell to 53-3 this year, broke Vinci three times in taking the first set after only 31 minutes.
Williams fell behind triple break point in the first game of the second set but held.
Vinci reached triple break point again in the fifth game and Williams, after her ninth and 10th aces of the match, fired a backhand wide to surrender the break and a 3-2 lead. Vinci held from there to force a third set.
After breaking for a 2-0 lead, Williams double faulted away a break to Vinci in the next game. She yelled between points often after falling behind to start the fifth game, in which she held.
"I was just trying to win points and win the match," Williams said.
Vinci broke to 4-3 when Williams swatted a forehand long, then denied Williams had two break points in the eighth game and held for a 5-3 edge.
Williams held but could not threaten Vinci in the last game, falling after two hours on the Italian's forehand drop volley winner.
Pennetta, 33, broke Halep in the fourth game and again in the sixth when the Romanian netted a forehand on the way to taking the first set in 28 minutes. She broke four times in the second set, a forehand winner capping her fourth win in five tries over Halep.