No one saw this coming.

Not with Serena Williams so close to tennis' grandest of feats, to one of the greatest achievements in any sport, on her home court at the U.S. Open no less.

Not even Roberta Vinci, the unheralded Italian on the other side of the net.

Asked if she thought she had any chance of pulling off the momentous upset, one that deserves to be ranked right up there with Buster Douglas knocking out Mike Tyson and the U.S. beating the Soviets in the Miracle on Ice, Vinci replied with admirable honesty.

"No," she said, shaking her head. "When I wake up, I say, 'OK, I am in the semifinals today. Try to enjoy it. Don't think about Serena.' I didn't expect that I (would) win."

But win she did, finishing it off with another of her deft little shots that skidded just out of Williams' reach.

With that, the Grand Slam was done.

The letdown could be felt far beyond Arthur Ashe Stadium.

Love her or hate her, Williams' quest to become the first player in 27 years to capture all four major championships — Australian Open, French Open, Wimbledon and U.S. Open — in a calendar year was one of those truly defining moments, the kind that most of us want to be part of in some small way even if we don't know the difference between deuce and a drop shot.

Now what?

"I just can't believe it," said 18-time major champion Chris Evert, speaking on ESPN's telecast but really speaking for us all.

Williams was in no mood to bare her soul when it was over.

This hurt is going to take some time to heal.

"I don't want to talk about how disappointing it is for me," she said at a terse post-match news conference that lasted barely three minutes. "If you have any other questions, I'm open for that."

One can't help but wonder how long it will take for Williams to bounce back from what is surely the most gut-wrenching defeat of her career. She remains just one major title behind Steffi Graf's 22, the most in Open era, so there's still an enticing goal to spur her on.

But time is not Serena's side. A couple of weeks shy of her 34th birthday, she can't afford to wallow for long in Bill Buckner-like misery.

Maybe if she had been beaten by another of the top women players, it would've been a bit easier to accept. But the 32-year-old Vinci was unseeded and playing in the semifinals of a major for the first time in her career.

After they split the first two sets, the Italian seemed to realize she had nothing to lose.

Williams, on the other hand, had everything to lose.

It showed.

Tied at three games apiece in the deciding set, Williams double-faulted twice. Then she uncorked a 126 mph ace — her fastest serve of the tournament — to get to deuce. Her last hurrah, it turned out. She hit the next one into the net, and then went long for another of what would grow to 40 unforced errors in the match.

The sport's greatest closer was overwhelmed by history, no matter much she tried to spin it a different way to reporters.

"I told you guys I don't feel pressure," Williams insisted. "I've never felt pressure."

Her play suggested otherwise.

As the match wore on, her legs seemed to get heavier and heavier. She kept getting caught flat-footed, struggling to get in the proper position to hit shots. Vinci, on the other hand, raced around the court, sending back everything Williams threw her way.

"In my mind I say, 'Put the ball on the court. Don't think about Serena in the other court. And run. Put the ball in the court and run,'" Vinci said.

"Then," she added, breaking into a big grin, "I won."

Williams' loss does nothing to diminish her amazing accomplishments. Few athletes have stayed at the pinnacle for such an extended period of time. No one in tennis, that's for sure. She won her first major title at the 1999 U.S. Open, just before her 18th birthday. Nearly 16 years later, she captured her 21st this past July at Wimbledon, capping a run of four straight major titles going back to last year.

A Serena Slam.

Not a Grand Slam.

"I felt really happy to get that win at Wimbledon," she said, trying to muster a smile. "I did win three Grand Slams this year. And, yeah, I won four in a row, so that's pretty good. So yeah, it's definitely a positive."

With that, she bolted for the door.

A few minutes later, she hopped into the backseat of a black SUV and was gone.

We all felt her pain.

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Paul Newberry is a national writer for The Associated Press. Write to him at pnewberry@ap.org or on Twitter at www.twitter.com/pnewberry1963