Today in U.S. Open: Serena Williams's Loss to Vinci Ends Grand Slam Bid

Serena Williams of the U.S. gets up slowly from the court in the third set against Roberta Vinci of Italy during their women's singles semi-final match at the U.S. Open in New York on September 11. Mike Segar/Reuters

Updated | In a major upset, Serena Williams lost to Italy's Roberta Vinci in the women's semifinals at the U.S. Open on Friday afternoon, thus ending her bid for a calendar-year Grand Slam.

The American tennis star, the No. 1 seed at the tournament played at the United States Tennis Association (USTA) Billie Jean King National Tennis Center in Flushing Meadows, New York, had won four straight major titles but needed the U.S. Open to achieve the calendar-year feat.

Vinci, who was unseeded in the tournament, will compete in the final on Saturday afternoon against another Italian, Flavia Pennetta. The No. 26 seed won the other women's semifinal match against Simona Halep of Romania, the No. 2 seed.

Also on the 12th day of the tournament Friday, the men's semifinals are scheduled to begin at 5 p.m. ET, two hours later than earlier planned becauase of Thursday's rain delays. Top seed Novak Djokovic of Serbia will compete against the defending champion, Marin Čilić of Croatia, and No. 5 Stan Wawrinka faces No. 2 Roger Federer; both are from Switzerland. The winners will compete in the men's final on Sunday evening.

Fans can check the schedule for the latest scheduling updates.

#Vinci on stopping history: "Today's my day, sorry guys!" #usopen

— US Open Tennis (@usopen) September 11, 2015

This story has been updated to include the results of the women's semifinals.