“After further consultation with medical staff, Simone Biles has decided to withdraw from the event finals for vault and the uneven bars,” the USAG statement said.
“She will continue to be evaluated daily to determine whether to compete in the finals for floor exercise and balance beam.”
Biles said on Friday she was struggling with the “twisties”, a mental block that has taken away her ability to perform the high-risk moves that she has trademarked.
Elsewhere on Saturday Dressel established himself as the dominant male swimmer of the Tokyo Games as he smashed his own world record to win the 100m butterfly title for his third gold medal.
The American touched in 49.45 seconds to better his own world best of 49.50 set two years ago.
Dressel could win another two gold medals, having already won the 100m freestyle crown and the 4x100m relay title with his team.
But he can no longer win six titles after the US team were beaten by Britain as the mixed 4x100m relay made its Olympic debut.
His US teammate Katie Ledecky stormed to her third straight Olympic 800m freestyle title with a dominant swim to bank her fourth medal in Tokyo.
Okagbare fails doping test
The morning action at the Olympic Stadium was overshadowed by Okagbare’s failed doping test.
The Nigerian won her women’s 100m heat on Friday but on Saturday the Athletics Integrity Unit said she had tested positive for human growth hormone in an out-of-competition test on 19 July.
The 32-year-old won the long jump silver medal at the 2008 Beijing Olympics.
US 400m hurdler Sydney McLaughlin said she believes her world record could go as she prepares for the latest instalment of her rivalry with 2016 Olympic champion Dalilah Muhammad.
On day two of the athletics programme, McLaughlin glided through her heat in 54.65 sec while Muhammad won hers in 53.97 on a track that is producing fast times.
Asked if she believed her world record of 51.90 could fall in Tokyo she replied: “I think anything is possible. It’s a really great group of girls so I wouldn’t put it past anybody.”
Pole vault world record holder Armand Duplantis of Sweden eased through qualification for a final missing his American arch-rival Sam Kendricks, who tested positive for coronavirus.
In the inaugural mixed triathlon, Britain’s Jonny Brownlee finally stepped out of his brother Alistair’s shadow by winning a gold medal as part of the victorious British quartet.
He finished in the silver medal position behind his elder sibling in the individual event at the 2016 Rio Olympics and took bronze behind him at London 2012.
“It feels absolutely amazing to complete the set, to have three Olympic medals, three different colours,” said Brownlee.
Belinda Bencic of Switzerland takes on Marketa Vondrousova of the Czech Republic for the women’s singles title later in a tennis competition that lost big names Naomi Osaka and Ash Barty in the earlier rounds.
Men’s world number one Novak Djokovic had his hopes of a first Olympic title crushed on Friday as he lost to Alexander Zverev of Germany 1-6, 6-3, 6-1 in the semi-final.
Zverev will face Russian Karen Khachanov in Sunday’s final.
In other action on Saturday, defending champions Brazil play Egypt in the quarter-finals of the men’s football tournament, while Spain face Ivory Coast.
And Li Junhui and Liu Yuchen can give China a second badminton gold in the men’s doubles final.