— -- NEW YORK -- Serena Williams has caught criticism over the years for contending that she beats herself more than opponents beat her. But maybe Friday's match will win her more believers. What became achingly clear throughout her US Open semifinal shocker against highly inspired Roberta Vinci -- the 300-to-1 shot who defeated her -- was Williams had been playing two opponents: herself and the unseeded Italian across the net who is ranked only 43rd in the world.

For Williams, it was as though the desire to get to Saturday's final and win it -- setting off the avalanche of history-making accomplishments that would've come -- seemed to make her feel as if she was swinging at the ball underwater or moving her feet in quicksand. Williams clearly wanted to accomplish the first calendar-year Grand Slam sweep since Steffi Graf did it in 1988. But she seemed to be clutching the racket too tightly; her two-fisted backhand was scattershot all day.

"I saw a frozen Serena Williams, I saw a paralyzed Serena Williams," ESPN analyst Chris Evert said.

Vinci's magnificently executed 2-6, 6-4, 6-4 win will go down as the greatest upset in tennis history because of the lofty records Williams was chasing. It's a shocker on par with Buster Douglas' knockout of Mike Tyson in Tokyo, or the 1980 U.S. Olympic hockey team's "Miracle on Ice" stunner over Russia.

What this loss will mean for Williams going forward is not destructive. Williams is 33 now, and she has to know it's unlikely she'll have another chance at the calendar-year Slam.

But beyond that? Her regard as the best female player in tennis history is already assured.

This setback will not create doubts about Williams heading into 2016, nor diminish how the astonishing year she just had is regarded when folks look back on it.

Williams has a lot of tread worn off the tires after 17 years on tour, it's true. But there's no reason to think she will not go on to tie Graf someday at 22 career majors and overtake Margaret Court, who holds the record of 24.

Her chase of them is not akin to a physically broken down, mentally washed out Tiger Woods looking less and less likely to break Jack Nicklaus' record for golf majors. The harder Woods tries to redo his swing or change coaches, the further behind he seems to fall. Williams is still a fearsome ball-striker. She remains the best server in women's tennis. She's a fierce competitor -- maybe too fierce.

Remember, we've seen her have this sort of nerve-jangling, teeth-gnashing trouble finishing off a career milestone before. It happened just last year, in fact.

Williams was trying to tie Evert and Martina Navratilova at 18 majors apiece. She actually arrived at the 2014 US Open openly mocking herself, saying she needed an attitude adjustment after she had fallen out of the 2014 Australian Open, then the French Open in May and Wimbledon in June all too early for her liking. She landed in New York saying she wanted to lighten up, keep things in better perspective, and constantly reminded herself that even if she never won another title, her place in history was assured.

Then she went out and won the whole damn thing -- and the next three majors to tee up her chase of history at this US Open a year later.

So forget the sight of Williams at her post-match news conference Friday, insisting "I told you guys, I never feel pressure." She absolutely does. And forget the sight of her batting away a later inquiry about whether her alternating moods in the final set -- when she was screaming "C'MON!" at the top of her lungs on some points and trying to remain calm and quiet the next -- were a hunt for some emotional equilibrium, some state of grace that would allow her to forget the nerves and play better.

"No," Williams again insisted. "I was just trying to win points at that time."

That's not all that was happening.

Williams was also locked in a nightmare Vinci, a career-long journeywoman, wouldn't let her out of.

Williams was on her way to becoming the latest tennis player who oddly has more trouble against inferior competition she's expected to crush than the near-equals that slug it out on near-even footing with her. Navratilova used to be the same way.

"She goes from arrogance to panic with nothing in between," tennis wit Ted Tinling once said of her.

Serena's Achilles' heel -- that almost intolerable need, not just wish, of hers to win, dominate, succeed -- is the same thing that makes her great. Even if she did fall one match short of the Slam sweep against Vinci, an opponent who conceded even she didn't think this upset was possible -- "No. Really. I didn't," Vinci insisted during her hysterically funny on-court interview right after the match -- Williams deserves the highest credit.

There's no diminishing the work and skill and effort and magic it took for Williams to get this far. Or the titanic achievement a Slam sweep asks of any player, even one as decorated as Serena. It's one of the toughest challenges in sports. It asks that you summon your best tennis on command for two-week stretches over a nine-month span at four venues around the world on three different surfaces (hard courts, clay and grass).

Nowadays, it also asks that you keep it up after you arrive in New York with the pressure and coverage crescendoing like never before.

Graf has admitted her 1988 chase exhausted her. Williams struggled with her emotions so much at her terse post-match news conference Friday that at one point she plaintively asked the roomful of reporters if anyone had a question not related to how disappointed she was.

But she also praised Vinci.

"I just think she literally played out of her mind," Williams said.

For a year now, Williams has, too. For her, Friday was a bad day. But the bad news for the rest of the tour is Williams will be back.