Serena Williams of the US waves to the crowd as she leaves the court after losing to Roberta Vinci of Italy on day twelve of the 2015 US Open tennis tournament at USTA Billie Jean King National Tennis Center in New York, Sept 11, 2015. [Photo/Agencies]

NEW YORK - Serena Williams' bid for a rare calendar-year Grand Slam came to a shocking halt at the US Open on Friday when the defending champion suffered one of the greatest tennis upsets to unseeded Italian Roberta Vinci.

The 32-year-old Vinci ended Williams' run of three straight US 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," Williams said about her 43rd-ranked opponent who was playing in her first grand slam singles semi-final. "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, 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.

"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."

Vinci, a winner of five grand slam doubles titles, took advantage of some loose points from the American, who appeared to feel the pressure of her pursuit of tennis history.

Vinci played with extraordinary composure and used her net-play skills to advantage with an inspired performance.