US Olympic star Simone Biles received support from fellow gymnasts, other athletes and celebrities after pulling out of the women's team event at Tokyo over mental health concerns.

Putting her mental struggles in front of the world saw Biles join a growing number of world-class athletes from across sport, such as Japanese tennis star Naomi Osaka and US swim legend Michael Phelps, who have opened up about coping with stress and the mental grind and toll of their success.

Among those supportive of Biles was Aly Raisman, who finished second to Biles in the all-around at the 2016 Rio Olympics.

"Just a friendly reminder: Olympic athletes are human & they're doing the best they can," Raisman tweeted. "It's REALLY hard to peak at the right moment & do the routine of your life under such pressure. Really hard."

Raisman and Biles were among the gymnasts who spoke up about former US gymnastics team doctor Larry Nassar, who was sentenced in 2018 for sexual abuse of athletes.

Asked about the pressure upon an athlete, Raisman told US Olympic telecaster NBC, "I feel sick to my stomach. It's just horrible."

She added, "It's just so much pressure," and said, "I think people forget sometimes we're human."

Biles had said she felt the "weight of the world" on her shoulders before taking part in a vault and pulling out of the team event, where her compatriots went on to take a silver medal.

"Simone, you've made us so proud. Proud of who you are as a person, teammate and athlete," tweeted Sarah Hirshland, US Olympic & Paralympic Committee chief executive officer.

"We applaud your decision to prioritize 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."

Aly Raisman (L, pictured at the Rio Olympics) was among many who offered support to US gymnast Simone Biles after she withdrew from the women's team event at the Tokyo Olympics, citing mental health concerns
Aly Raisman (L, pictured at the Rio Olympics) was among many who offered support to US gymnast Simone Biles after she withdrew from the women's team event at the Tokyo Olympics, citing mental health concerns AFP / Toshifumi KITAMURA

Retired US gymnast Kerri Strug, whose clutch vault with an ankle injury helped power the US women's team to gold at the 1996 Atlanta Olympics, tweeted: "Sending love to you @Simone_Biles -- Team UNITED States of America," with a heart and an emoji goat, to stand for "Greatest Of All Time."

While such claims add to the pressure in some ways, they also summed up how many Americans felt about Biles before and after her decision to withdraw from the team event.

"Gratitude and support are what @Simone_Biles deserves," tweeted White House spokeswoman Jen Psaki.

"Still the GOAT and we are all just lucky to be able to see her in action. And congrats and tks to the tough as nails #TeamUSA @sunisalee_ , @ChilesJordan, @grace_mccallum2 for being role models and champs," she added, tagging Biles's teammates Sunisa Lee, Jordan Chiles and Grace McCallum.

NBA Minnesota Timberwolves forward Karl-Anthony Towns tweeted: "Sending nothing but love and positivity to the (GOAT) @Simone_Biles."

Two-time Olympic Alpine ski champion Mikaela Shiffrin offered support as well, tweeting: "@Simone_Biles keep whipping out that smile of yours cause it is undeniably golden. Always."

Filipino boxing legend Manny Pacquiao tweeted: "Once a champion, always a champion. God Bless @Simone_Biles."

Adam Rippon, who helped the US figure skating squad to a bronze medal at Pyeongchang in 2018, also backed Biles, tweeting, "I can't imagine the pressure Simone has been feeling. Sending her SO much love. (It's) easy to forget she's still human. WE LOVE YOU."

US gymnast Morgan Hurd, the 2017 world all-around champion, tweeted: "So insanely proud of this team. no one but them knows how hard this sport is not only physically but mentally and they handled everything with grace. couldn't be more proud, love you guys."