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Serena Williams during the penultimate game of her three-set semifinal loss to Roberta Vinci. Williams had never lost a set to Vinci before Friday’s match.Credit Chang W. Lee/The New York Times

After losing in the United States Open semifinal to Roberta Vinci, an unseeded Italian, Serena Williams should have exhaled with such force that the grandstands surrounding her rumbled.

Because it was finally over, wasn’t it? The hype and the hope and the heavy book of tennis history that were hovering over her, threatening to crush her with expectations that she should win the first Grand Slam in 27 years, finally had been lifted. Just like that, on Friday in Arthur Ashe Stadium, all of that pressure was let loose by a shocking upset that ended with a forehand from Vinci that bounced too far away for even the superhero Williams to return it.

When Steffi Graf won the Grand Slam in 1988, at 19, she said she was more relieved than happy because the burden to win had been so oppressive. But Williams, postmatch, dared not show emotion or offer much explanation. No tears, anger or visible release after losing, 6-2, 4-6, 4-6, to a player who was 32, had never even taken a set off her before, and who had never even reached a Grand Slam final. A player who hadn’t even existed in the stratosphere that Williams dwells in — until she ascended there Friday, when it counted most.

After being criticized and praised and vilified and canonized over the past weeks and months, Serena Williams, queen of tennis and the world’s No. 1 player, was not about to let anyone know the details of what she was thinking. It was her prerogative, of course, but so many of her fans could have empathized with her much more, and understood her much more, if she had just let them in.

Instead, Williams, 33, just grabbed her bag, waved to the crowd and walked off the court. Later, she refused to answer a question regarding her disappointment.

“I’m not — I don’t want to talk about how disappointing it is for me,” she said, speaking in a low tone. “If you have any other questions, I’m open for that.”

How about pressure? There was so much for her to handle, right?

“I told you guys, I don’t feel pressure,” she said. “I never felt pressure. I never felt that pressure to win here. I said that at the beginning.”

2015 U.S. Open

It would have been more believable for her to say that she didn’t want to talk about it.

As the matches at the Open went on, Williams looked more miserable and sadder, capped this week by her victory over her elder sister Venus in the quarterfinals. After that win, she snapped at a reporter, who had asked why she wasn’t smiling, “To be perfectly honest with you, I don’t want to be here.”

Was it pressure? Maybe. By Friday, Vinci saw firsthand that Williams was feeling the pressure. At one point in the match, Vinci began to smile as Williams started talking to herself on court, shouting and attempting to pump herself up by grunting louder and telling herself, “C’mon!” or saying, “It’s OK.”

Aha, Vinci thought from across the net, Williams — the winner of 21 Grand Slam singles titles — is mortal after all. She saw Williams smash her racket after Vinci had won the second set. And she watched Williams double-fault in the third set. It wasn’t a stretch to say that the often-stoic and always fierce Williams was cracking, with chunks of her previously impenetrable shell chipping off, bit by bit, onto the Open’s blue courts.

“In my mind, I say: ‘Think about this. She’s nervous,’ ” Vinci said. “ ‘So try to keep it and fight every single point.’ ”

Nerves were the difference in the match. Williams had 16 aces, but she made 40 unforced errors and, at times, seemed flummoxed by Vinci’s crafty play. There were moments when she couldn’t keep up with Vinci’s fleet footwork and couldn’t reach drop shots that lost speed so quickly it was as if the balls had turned to lead, midair. It turned into an unexpectedly thrilling matchup between two players in their 30s, nearing the end of their careers. But the two couldn’t be more different.

There was Williams, one of the most recognizable athletes in the world, trying to trick herself into thinking there was no pressure. And there was Vinci, virtually unknown to everyone in New York before Friday, who had persuaded herself to have fun. Soak up the crowd and the atmosphere, she had told herself beforehand. Pretend Williams isn’t even there. Ignore that Williams is going after a Grand Slam.

After one big point in the third set, Vinci put her right hand to her right ear and then raised her arms to get the pro-Williams crowd behind her.

“When I wake up, I say, ‘O.K., I have a semifinal today, try to enjoy, don’t think about Serena, play, enjoy, enjoy,’ ” she said in an interview after the match, during which she smiled, gestured, giggled and — in one fell swoop — epitomized the joy of sports and unexpected outcomes.

She said that she never dreamed that beating Williams was possible, and that her goal was to simply “put all the ball on the court, don’t think about that Serena is in the other court, and run!”

“You put the ball and run,” she said. “Don’t think, and run. And then I won!”

Even fans devastated by Williams’s loss would have been hard-pressed to keep from grinning as Vinci described what she called the best day of her life.

She apologized for winning, but not for doing something incredible. So incredible, she said, that she now could even “touch the sky with my finger.”

“For the American people, for Serena, for the Grand Slam and everything,” she said, explaining why she felt bad for winning. “But today is my day. Sorry, guys.”

Williams didn’t apologize. She did give Vinci credit for “playing out of her mind,” especially at what she said was “the late age” of 32. “Actually, I guess it’s inspiring,” Williams said.

She was looking down when she tried to explain the positives of all of this. “I won four in a row,” she said of her recent Grand Slam victories. “It’s pretty good.”

Now, if she would just let herself breathe out and give herself some time to reflect, she would realize that her accomplishments this year have been much more than pretty good. They’ve been phenomenal. And nobody has to guess that’s inspiring.

Even before this Open began, before the pressure to win a Grand Slam boiled over on Friday, Serena Williams was already one of the greatest ever. Losing on Friday didn’t change that. Winning the Grand Slam wouldn’t have changed that, either.