The 32-year-old Vinci ended Williams' run of three straight U.S. Open titles and her bid to become the fourth woman to sweep all four grand slam singles titles in the same year with a 2-6 6-4 6-4 win that stunned the Arthur Ashe Stadium crowd.
"I thought she played the best tennis in her career," said 33-year-old Williams. "You know, she's going for it at a late age.
"Actually, I guess it's inspiring. I think she played literally out of her mind."
For world number one Williams, whose remarkable streak of 33 consecutive match wins in the grand slams was also snapped, it marked the first time she lost to Vinci in five career meetings after a string of easy straight-set wins against her.
Vinci's extraordinary win set up an all-Italian women's final on Saturday against 33-year-old Flavia Pennetta, who routed second seed Simona Halep of Romania 6-1 6-3.
The 46th-ranked Vinci, appearing in her first grand slam singles semi-final, took advantage of some loose points from the American, who appeared to feel the pressure of her pursuit.
Vinci, a winner of five grand slam doubles titles, played with extraordinary composure and used her net-play skills to advantage with an inspired performance.
"This is an incredible moment for me," Vinci said after taking a long moment to compose herself for the on-court interview. "It's amazing. It's like a dream. I'm in the final. I beat Serena."
(Reporting by Larry Fine; Editing by Frank Pingue)
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