"It's like fighting all those demons and coming out here. I have to put my pride aside. I have to do it for the team," she told reporters after the team competition, in which her teammates stepped up and took silver.
"And then at the end of the day, it's like 'you know what, I have to do what is right for me and focus on my mental health and not jeopardise my health and my well-being,'" she said.
'WE LOVE YOU'
Olympians, athletes, other US figures were quick to rally around Biles after her decision.
"Simone, you've made us so proud. Proud of who you are as a person, teammate and athlete," said Sarah Hirshland, the Chief Executive of the US Olympic and Paralympic Committee. "We applaud your decision to prioritise your mental wellness over all else, and offer you the full support and resources of our Team USA community as you navigate the journey ahead."
White House Press Secretary Jen Psaki wrote on Twitter: "Gratitude and support are what [Biles] deserves. Still the GOAT and we are all just lucky to be able to see her in action."
Former US figure skater Adam Rippon empathised, tweeting: "I can't imagine the pressure Simone has been feeling. Sending her SO much love. It (sic) easy to forget she's still human. WE LOVE YOU."
Danusia Francis, a British-born gymnast representing Jamaica, said Biles’ decision was empowering.
"Don't know about you but I think @Simone_Biles just empowered everyone to put their mental well-being above everything else. WHAT A QUEEN. GOAT in more ways than one," she tweeted.
Messages of support also came from UNICEF USA, champion boxer Manny Pacquiao and Olympic gold medal-winning alpine skier Mikaele Shiffrin.
After the vault, Biles had a hasty conference with her team at the side of the runway, looking upset, before walking off the floor with her bag. She reappeared in her warm-up suit, watched her teammates snag silver, and even clowned around dancing with fellow teammate Jordan Chiles.
But later she spoke of feeling "lost" after the vault and deciding that she needed to "call it," stressing that she made the decision and not her coaches.
She has kept open the possibility of still competing in Tokyo, but said she would take it a day at a time. Individual all-around and apparatus competitions still remain.
By stepping back, and speaking up about mental health, Biles joins Japanese tennis star Naomi Osaka, who earlier this year dropped out of the French Open, citing the need to prioritise her mental health over taking part in mandatory news conferences after she was fined for not appearing.
Osaka lost in the tennis earlier on Tuesday and said she too had felt the weight of expectations. She was the final torchbearer to light the Olympic cauldron at Friday's opening ceremony.
"I definitely feel like there was a lot of pressure, this time around," Osaka said. "I think it's maybe because I haven't played in an Olympics before and for the first one to be here was a bit much."
"But I think I'm glad with how I played... with taking that break that I had. I've taken long breaks before and I've managed to do well," Osaka said.
Readers seeking support with mental health can contact Beyond Blue on 1300 22 4636 . More information is available at Beyondblue.org.au. Embrace Multicultural Mental Health supports people from culturally and linguistically diverse backgrounds.
LGBTIQ+ Australians seeking support with mental health can contact QLife on 1800 184 527 or visit qlife.org.au. ReachOut.com also has a list of support services.