Serena Williams' quest to win a first calendar Grand Slam was sensationally ended on Friday when she was beaten by Roberto Vinci in the semi-finals of the US Open.

The legendary American looked to be on course to become the first player since Steffi Graf in 1988 to win all four majors in the same year when she claimed the first set against the unseeded Vinci at Arthur Ashe Stadium.

But unheralded Italian Vinci took advantage of a nervy performance from the out-of-sorts world number one, who appeared to freeze with so much at stake and was beaten 2-6 6-4 6-4.

Vinci, who had never been beyond the fourth round of a major before her exploits at Flushing Meadows this week, will now face compatriot Flavia Pennetta in Saturday's final after pulling off one of the most stunning upsets of all time.