Unseeded Italian Roberta Vinci has spectacularly ended tennis world No.1 Serena Williams' quest for an elusive calendar-year grand slam with a semi-final boilover at the US Open in New York.
Ranked 43rd in the world and contesting her first grand slam semi-final, Vinci stunned the top-seeded three-time defending champion 2-6 6-3 6-4 to set up an improbable all-Italian title match with Flavia Pennetta.
Pennetta, the 26th seed, earlier upset world No.2 Simona Halep 6-1 6-3 in today's first semi-final to also progress to her maiden grand slam singles final.
Unbeaten without ever losing a set against Vinci in four previous meetings, Williams had been riding a 33-match grand slam winning streak in a seemingly relentless pursuit of a record-equalling 22nd career major.
But after charging through the opening set in 31 minutes, Williams succumbed to unimaginable nerves - and an inspired Vinci, who admitted she thought it wasn't possible to topple the all-conquering top seed.
The reigning US, Australian and French Open and Wimbledon champion had been bidding to become the first player - man or woman - to win all four grand slam singles crowns in a season since Steffi Graf in 1988.
Vinci apologised to the fans for ending Serena's grand slam bid.
"I am sorry guys, but today is my day," she said.
"This is like a dream. I beat Serena, I'm in a grand slam final. It's the best moment of my life."
Williams also missed the chance to equal Graf's open-era record of 22 singles majors and draw to within two of Australian Margaret Smith Court's all-time benchmark.
Eerily, Williams' last defeat at Flushing Meadows also came on 9/11 - the 10th anniversary of the attacks no less - to Australian Samantha Stosur in the 2011 final.
"I don't want to talk about how disappointing it is for me."
"I thought she played the best tennis of her career," Williams said.
"At 33, she's going for it I think she played literally out of her mind."
"I don't think I played that bad. I made a few more unforced errors than I normally make."
"She just didn't want to lose today. Neither did I."
Pennetta, who heads Vinci 5-4 in head-to-head meetings, earlier continued her own giantkilling run to reach her maiden grand slam final in just 59 minutes.
Pennetta crunched 23 winners to Halep's six and won 19 of the last 21 points of the match after falling behind 3-1 in the second set.
Apart from taking out Australia's former champion, Pennetta upset two-time Wimbledon winner and fifth seed Petra Kvitova in her sixth quarter-final in New York.
Contesting her 49th career major, Pennetta said it meant "everything" to finally make a grand slam final.
"I didn't expect to make it this far," she said.
"I just try to play my best every match. I'm taking it match by match."
"I'm playing very well and it's amazing to be here."