New York, Sep 12 (IANS/EFE): Unheralded Italian Roberta Vinci pulled off one of the biggest upsets in tennis history here on Friday, rallying to defeat Serena Williams 2-6, 6-4, 6-4 in the US Open semifinals.

The 43rd-ranked Vinci, who is best known for her achievements in doubles and had never won a set in four previous matches against the 21-time Grand Slam champion, will next take on 26th-seeded countrywoman Flavia Pennetta in tennis' first-ever all-Italian major singles final.

The 33-year-old Pennetta advanced to the championship match by routing second-seeded Romanian Simona Halep 6-1, 6-3 earlier Friday.

Vinci's low-bouncing slice backhand and strong serve for her height were her main assets, while the pressure Serena faced in trying to become only the sixth player in tennis history - after Don Budge, Maureen Connolly, Rod Laver, Margaret Court and Steffi Graf - to win all four Grand Slam singles titles in the same calendar year proved to be a big equalizer.

In the battle of WTA Tour veterans, 34-year-old Williams used her big edge in the power department to strike 50 winners, compared to just 19 for the 32-year-old Vinci.

But the American's 40 unforced errors - double the number of mistakes the unseeded Vinci committed when not under pressure by the world No. 1 - proved to be her undoing.

Although she fell short in her bid for the calendar-year Slam, Williams' victory at Wimbledon earlier this summer made her the holder of tennis' four biggest events - the Australian Open, the French Open, Wimbledon and the U.S. Open - for the second time in her career.