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TOKYO – She's been called superhuman. Otherworldly. Invincible. One word, though, has never come to mind when discussing Simone Biles: vulnerable.

For the past five years, it seemed nothing could ground the greatest gymnast of all time. The weight of others' expectations became too great for Biles to bear on Tuesday, at the worst possible time. The U.S. had just begun competing in the team finals at the Tokyo Olympics when Biles decided she could not continue, saying she was not in the right "head space" to perform her high-risk gymnastics safely and effectively.

At a time when athletes have begun discussing mental health concerns more openly, Biles' pain was laid bare on the worldwide stage of the Olympic Games. She left the floor at Ariake Gymnastics Centre after landing awkwardly on a vault, nearly sitting down and then lunging forward. Before the next rotation, Biles reappeared and spent the rest of the competition alongside her teammates, watching them win the silver medal behind Russia.

Like other athletes, including tennis player Naomi Osaka, swimmer Michael Phelps and fellow Olympic gymnast Sam Mikulak, Biles has spoken candidly about the emotional struggles faced by world-class competitors. She did so again Tuesday, a superhuman athlete sharing her very human anguish.

"I know that I'm not having as much fun," Biles said, her voice breaking. "And I know that this Olympic Games, I wanted it to be for myself. I came in, and I felt like I was still doing it for other people.

"So that just hurts my heart, that doing what I love has been kind of taken away from me, to please other people. … I have to do what's right for me and focus on my mental health."

Early Wednesday morning, USA Gymnastics tweeted that Biles would be withdrawing from Thursday's individual all-around finals. She had qualified in all four individual events.

The hype surrounding Biles has been relentless during the buildup to the Tokyo Games. She is on magazine covers and in ubiquitous TV commercials on NBC. A documentary series about her life, "Simone vs. Herself," is now airing.

On social media, Biles gave a hint Sunday of how she was feeling after the qualifying round. She made several uncharacteristic mistakes in that competition: hopping all the way off the floor on the landing of a tumbling pass, stepping sideways off the mat on a vault landing, taking three big steps back on her beam dismount.

On Instagram, Biles posted, "I truly do feel like I have the weight of the world on my shoulders at times. I know I brush it off and make it seem like pressure doesn't affect me, but damn, sometimes it's hard."

That's what she tried to do Tuesday: Brush it off. Biles was having trouble executing some of her skills during practice in the back gym, to the point where teammate Jordan Chiles said Biles was "giving us a little heart attack." In the hours before the competition, Biles said she was shaking and restless, a feeling she had never had before a meet.

Typically, Biles would push through, knowing her team needs her and fans want to see her. Tuesday, she simply could not. The skills Biles does can be dangerous if she has any doubts or fears or distractions, and once she recognized she was "fighting with her own head," she didn't want to risk injury or hurt her team's score.

"It's like fighting these demons, and then coming out here," she said. "Like, I have to put my pride aside, I have to do it for the team.

"I knew I had to take a step back. Four or five years ago, I would have gone out there and done whatever. But today, it was like, 'You know what? No, you're not.' "

The U.S. Olympic & Paralympic Committee (USOPC) has recently put more emphasis on mental health support for its athletes. The organization launched a mental health task force last year, hired a director of mental health services, added staff and increased athlete education.

Several of those people are in Tokyo to assist athletes during the Olympics, perhaps the most pressure-packed competition they will ever face.

"We've looked at what resources need to be in place for athletes when they do have challenges," said Bahati VanPelt, the USOPC's director of athlete services. "Athletes need comprehensive support. These are first-time, dedicated wellness resources for Team USA."

Biles said she knows resources are available, but she still fights the urge to push on. She is grateful to other athletes for speaking up about their own vulnerabilities and advocating for themselves.

Biles said she was inspired by a Netflix documentary series about Osaka, who has spoken about battling depression and anxiety and bowed out of the French Open to protect her mental health.

In a statement late Tuesday, Sarah Hirshland, CEO of the USOPC, said the organization was proud of Biles as a person, teammate and athlete. "We applaud your decision to prioritize your mental wellness over all else," the statement said, offering Biles the "full support and resources of our Team USA community as you navigate the journey ahead."

Like Osaka, Phelps and others, Biles has become comfortable discussing a topic that can be uncomfortable, especially for high-level athletes.

"At the end of the day, we're human, too," she said. "We have to protect our mind and our body, rather than just go out there and do what the world wants us to do."