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In a stunning upset, Serena Williams was bounced from the U.S. Open on Friday and lost her bid for the first Grand Slam in tennis in 27 years.

Roberta Vinci, an unseeded Italian ranked No. 43 in the world, beat Williams 2-6, 6-4, 6-4 in the semifinals.

Williams was aiming for her 22nd major title, which would have tied her with Steffi Graf for the most in the Open Era. She had won the Australian Open, the French Open and Wimbledon and needed only the U.S. Open to clinch the Grand Slam.

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Vinci, 32, had never made it past the quarterfinals of a Grand Slam. In four previous matches against Williams, she had never even won a set.

"She played literally out of her mind," Williams said.

On court after the match, Vinci called it the greatest moment of her life.

"It's an incredible moment for me," she said. Told by an ESPN reporter that she was a 300-to-1 underdog against Williams, she said, "Really?", then repeated it in Italian to her coach.

She apologized — "for the American people, for Serena, for the Grand Slam." But she said, "Today is my day. Sorry, guys."

Williams had not lost a Grand Slam match since June 27, 2014 — and not at the U.S. Open in four years. The loss left her agonizingly short of what would have been the first Grand Slam since Graf in 1988.

Her bid was the most hotly anticipated event in tennis in years. Tickets to the women's final sold out days before the first match, and it was the first time in memory that the women's final had sold out before the men's.

Vinci will play the 26th seed, Flavia Pennetta, in the final on Saturday.