Williams looked on course to continue her quest to become the first player to claim all four grand slam titles in the same year since Steffi Graf in 1988 after comfortably winning the first set.
But unseeded Italian Vinci worked her way into the contest to pull off a 2-6 6-4 6-4 win in front of a stunned crowd at Arthur Ashe Stadium on Friday to reach her first grand slam final at the age of 32.
Williams made 40 unforced errors as she appeared to freeze with so much at stake and Vinci, who admitted post-match she didn't think she could beat Williams, took full advantage to ensure she'll face compatriot Flavia Pennetta in Saturday's final.
Speaking in her post-match press conference a seething Williams angrily deflected questions from the media suggesting the occasion had gotten the better of her.
"I don't want to talk about how disappointing it is for me," she quipped.
"I thought she played the best tennis of her career. I think she played literally out of her mind.
"I don't think I played that bad. I made more unforced errors than I normally would make, but I think she just played really well.
"I don't feel pressure, I've never felt that pressure to win here, I said that from the beginning.
"I made a couple of tight shots, maybe just about two, but for me that is just a normal match to make two tight shots, other than that I don't think I was that tight."