Serena Williams (right) loses to Roberta Vinci in the semifinals of the U.S. Open Sept. 11, 2015, ending her bid to win a calendar-year grand slam, a feat that hasn’t been accomplished in 27 years.
ABC 7 screenshot

A stunned Serena Williams quickly ran off the court after her dreams of pulling off a calendar-year grand slam—a feat that hasn't been accomplished since Steffi Graf in 1988—were dashed when Williams was bounced from the U.S. Open Friday by a 300-1 underdog.

According to NBC News, little-known Roberta Vinci, 32, an unseeded Italian ranked No. 43 in the world who had never made it past the quarterfinals of a grand slam, beat the No. 1 player in the world 2-6, 6-4, 6-4.

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Williams had already won the Australian Open, the French Open and Wimbledon. The last tournament to win to complete the grand slam was the U.S. Open.

Shortly after her loss, Williams told reporters at a news conference that she thought she played well and added that she didn't feel the pressure of playing for a grand slam. Williams noted that there were only two shots where she felt she was "tight" but gave the credit for her loss to her opponent, adding that Vinci played "out of her mind."

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"It's an incredible moment for me," Vinci said after the match. She apologized to those who wanted to see Williams complete the task that hadn't happened in 27 years.

"For the American people, for Serena, for the grand slam," she said, explaining her apology, before adding, "Today is my day. Sorry, guys."

Read more at NBC News.