Serena Williams suffered one of the greatest shocks in tennis history as unseeded Italian Roberta Vinci ended the American's hopes of a calendar grand slam with a stunning victory in the US Open semi-finals.

Vinci, ranked 43rd in the world, had never gone further than the quarter-finals of a major tournament before but the 32-year-old came from a set down to beat Williams 2-6 6-4 6-4.

It means Williams' run of 33 consecutive grand slam victories comes to an end while Vinci will now play fellow Italian Flavia Pennetta in Saturday's final.

After wrapping up her momentous win, Vinci broke down in tears, dumbfounded as she struggled to speak during her on-court interview.

"It's an incredible moment for me," Vinci said.

"It's like a dream, I'm in the final, I beat Serena, it was an incredible match. When I served I was like voooooo, it was impossible.

"I tried to stay focused, not to think about the match or about Serena, she is an incredible player. It is the best moment of my life."

Vinci added: "I am sorry for the American people, for Serena, for the grand slam, but today is my day, sorry guys."

Vinci had been knocked out in the first and second rounds at all three major tournaments this year but after a slow start, the wily Italian unsettled her opponent with slice, angle and variety of shot.

Williams opened up with an ace, one of 16 she served in the match, but four baseline errors gave Vinci an early break and a 2-1 lead.

The top seed switched her racket at the changeover and it seemed to do the trick, as Williams broke back, winning five out of the next six games to take the set with ease.

Vinci, however, refused to be discouraged as she snatched the first break again in the second set and this time held on, surviving a break point at 5-4, to serve out and force a decider.

Williams vented her frustration, bashing her racket at the changeover for which she picked up a warning from the umpire, but she seemed to recover as she stormed into a 2-0 lead.

Vinci, however, would not lie down and a Williams double fault handed the break straight back to the Italian, who was growing in confidence with every point won.

Williams cranked up the volume at 2-2, greeting every point with a violent scream and clench of the fists, but two games later Vinci responded in kind, cupping her ear to the crowd and pointing to her chest after finishing a scintillating point with a volley.

The drama was far from over as Vinci hustled, harried, chipped and chiselled her way to a break of serve at 3-3 and then held serve to sit one game away from victory.

Williams held to put the pressure back on her opponent but Vinci showed nerves of steel at 5-4, producing two superb half-volleys to serve out and complete the most unlikely of triumphs.