For world number one Williams, who was bidding to become only the fourth woman to win all four majors in the same year, it marked the first time she lost to Vinci in five career meetings.
Vinci, appearing in her first grand slam singles semi-final, took advantage of some loose points from the American with inspired play and determination to snap Williams’ streak of 33 consecutive match wins in the grand slams.
The 32-year-old Italian ended the two-hour shocker with a half-volley winner to set up a most unlikely Flushing Meadows final against 33-year-old compatriot Flavia Pennetta, who beat Romanian second seed Simona Halep 6-1 6-3 to reach her first grand slam singles final.
