Roberta Vinci stuns Serena Williams in US Open semis

In perhaps the most stunning upset in women's tennis history, Roberta Vinci took down Serena Williams in the US Open semifinals Friday, 2-6, 6-4, 6-4, stopping Williams two wins short of a calendar Grand Slam.

Somehow, someway, the plucky unseeded Italian - who at 32 was playing in her first major semifinal - managed to seize the moment, while the 21-time Slam champion was reduced to a bundle of nerves.

Vinci has had a phenomenal doubles run in the past three years, winning four majors and making the finals at three others, but she's never quite managed to bump her singles career out of neutral. She has a quirky, unconventional game that's short on power and features a backhand that she almost exclusively slices.

In this match, her oddball variety seemed to throw the world No. 1 for a loop. She consistently managed to reset points with those defensive slices, batted back sharp-angled balls that looked like sure winners, busted out dying drop shots that Williams was slow to read and react to, hit topspin lobs that found the baseline, and calmly landed stabbing volleys at the net.

Williams cruised through the first set and change, overcoming a small early hiccup by winning six straight games, and seemed well on her way to a fifth straight US Open final. But a strange thing happened in the second. Vinci, who it seemed Williams had expected to roll with the punches, dug in and started to punch back.

Williams, who's been error-prone all tournament, noticeably lost faith in her strokes, especially her backhands, which she stopped swinging through and started rolling in, extremely short and with hardly any pace.

Even after Vinci held her nerve and served out the second set, the first she'd taken in five meetings with Williams, there was little to suggest this was where the calendar Slam bid would end. Nineteen times Williams had played a deciding set in 2015, and 18 times she'd won, including all 11 at the majors. She may have been experiencing some yips, but there are few tasks in all of sports more difficult than closing her out, let alone for a doubles specialist who'd never been past the quarters in a majors singles draw through her 47 previous tries.

After Williams seemed to set things in order with a break right off the bat in the third, Vinci again refused to go away, breaking right back and giving the defending champ a dogfight she never signed up for. Vinci, who did not even have listed odds to win the tournament before it began. Vinci, who'd had to qualify for the tuneup in New Haven just two weeks prior.

Once it became clear Williams' 33-match Grand Slam winning streak was in serious jeopardy, once Vinci's happy-just-to-be-here looseness turned into legitimate belief, the whole scene turned surreal.

Williams had overcome deficits, enormous pressure, nerves, illness, injury, and everything else under the sun just to be where she was. By match's end, she looked emotionally and physically exhausted. She looked ready for it be to over.

And when Vinci stuck a deft little half-volley winner on match point, it was. A historic run for Williams. The match of a lifetime for Vinci. It was over.

And so, there will be no calendar Slam. Not this year. An improbable, all-Italian US Open final awaits.

"I'm sorry for the American people, for Serena, the Grand Slam, everything," Vinci said after the match. "But today is my day."

Yes, indeed.