Simone Biles has pulled out of two more Olympic finals at the Tokyo Games, USA Gymnastics said in a statement on Saturday, with her entire campaign now in serious doubt.

The 24-year-old gymnastics great came to Tokyo seeking five gold medals to equal the Olympic all-time career record of nine, but withdrew during the women's team competition and also skipped her all-around title defence, citing mental health issues.

That left Biles, who won four gold medals at the 2016 Rio Games, with a potential four more medal events in Tokyo, but she pulled out of two of those on Saturday.

"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", the mental block that has sidelined her in Tokyo.

The US federation said MyKayla Skinner would compete in the vault finals alongside Jade Carey.

"We remain in awe of Simone, who continues to handle this situation with courage and grace, and all of the athletes who have stepped up during these unexpected circumstances," the statement said.

Biles posted video on Instagram on Friday of her landing on her back in training, on cushioned pads, and wrote that she was having problems "literally on every event, which sucks".

She said previous bouts of 'twisties' had taken two or more weeks to pass, but there was "honestly no telling (the) time frame".

Biles, who was bombarded with messages of support after her earlier pull-outs, also hit back at critics, saying: "For anyone saying I quit, I didn't quit. My mind and body are simply not in sync as you can see here."

Biles won a silver medal in the women's team event because she started the final.

Britain wins mixed 4x100m relay gold
Adam Peaty's British team smashed the world record to win the inaugural Olympic 4x100m mixed medley relay gold on Saturday, outgunning China and Australia.

Britain's four-strong team of Kathleen Dawson, Peaty, James Guy and Anna Hopkin touched in 3min 37.58sec to break the previous 3:38.41 mark set by China in Qingdao last year.

The Chinese were second in 3:38.86 and Australia third in 3:38.95. The Caeleb Dressel-led United States were relegated to fifth.

The mixed relay was one of three swimming events debuting in Japan along with the men's 800m freestyle and women's 1500m freestyle.

Teams comprised two women and two men, with each of the four swimmers allocated to one of the four traditional medley strokes -- backstroke, breaststroke, butterfly and freestyle.

Britain chose to lead off with Dawson, up against American backstroke giant Ryan Murphy and Australia's Kaylee McKeown, who won the 200m individual title minutes earlier.

Peaty, who won 100m breaststroke gold in Tokyo, then took up the baton.

Guy swam a storming butterfly leg to hand over to Hopkin, who brought it home ahead of China's Yang Junxuan, Australian 100m freestyle champion Emma McKeon and a chasing Dressel.

McKeown wins women's 200m backstroke gold
Australian swimming sensation Kaylee McKeown completed an Olympic backstroke double on Saturday, adding the 200m title to her 100m crown.

The 20-year-old swam a blistering final 50m to touch in 2min 04.68sec ahead of Canada's Kylie Masse (2:05.42), whom she also edged into silver over the 100m showdown.

Seasoned Australian campaigner Emily Seebohm (2:06.17) took bronze in her fourth Olympics.

McKeown paced her race perfectly, narrowly behind Masse until 150m when she put her foot to the floor to comfortably win gold.

McKeown, the 100m world record holder, was always favourite, being almost two seconds faster than any other swimmer in the field heading into the Games, recently clocking the third-quickest time in history.

US hurdlers sparkle at Olympics
The good and the ugly side of Olympic athletics were on show on Saturday as Sydney McLaughlin and Dalilah Muhammad sparkled on the track while Nigerian sprinter Blessing Okagbare was thrown out of the Games after failing a doping test.

McLaughlin and Muhammad, her predecessor as women's 400 metres hurdles world record holder, should put up as eye-catching a duel as the potential clash between Karsten Warholm and Rai Benjamin in the men's race.

Okagbare by contrast will not be lining up in the women's 100 metres semi-finals later on Saturday after it was announced she had tested positive for human growth hormone.

Both McLaughlin and defending champion Muhammad eased through their heats.

McLaughlin said her world record of 51.90sec could be under threat due to a fast track at the Tokyo Olympic Stadium and the overall quality of the field.

"It feels pretty fast, I must say," said the 21-year-old American.

"You can feel the bounce. Some tracks just absorb your bounce and your motion, this one regenerates it and gives it back to you.

"I think anything is possible. It is a really great group of girls, so I wouldn’t put it past anybody to go out and do it (break the world record)."

Muhammad has not had the smoothest of preparations but her ability to rise to the occasion of a major championships could give her an edge. 

"I am feeling good now, Covid is thankfully behind me and the injuries that followed," she said.

"So, I am feeling good and ready to run."

Whilst McLaughlin's dreams of gold remain firmly on course, British hurdler Jessie Knight's campaign did not get past the first hurdle.

The 27-year-old lost her balance before the hurdle and crashed into it -- once back on her feet she put her hands to her face and trudged off in tears.

- 'A mimic ' -
If there is to be a challenge to the American duo it is likely to come from Femke Bol.

The 21-year-old Dutch athlete put in an assured performance in her heat, cantering home well clear of her rivals.

“I love the track and these conditions. I am feeling well, sleeping well and am in great shape," said Bol.

“It is great to be in this shape coming into this tournament."

The USA also hold a strong hand in the 100m hurdles with world record holder Kendra Harrison praying that this time she ends her major championship jinx.

The 28-year-old failed to qualify for the Rio Games, was fourth in the 2017 world final and had to settle for silver behind compatriot Nia Ali at the 2019 world championships.

"I'm just trying not to emphasise the word 'Olympics' and just trying to take it like I do every meet, and not to put pressure on myself," said Harrison.

"I'm going to give it everything I've got. It's semis, a mimic of the final."

Injuries have robbed Kenyan great David Rudisha of a shot at a 'threepeat' in the 800 metres.

Nijel Amos, the man who took silver behind Rudisha when he broke the world record in the 2012 final, has battled with injuries but he looked mighty smooth in his heat.

American champion and 2016 Olympic bronze medalist Clayton Murphy is another who believes he is in a better place than a couple of years ago when he finished stone last in the world final.

The rough and tumble of the 800m played out badly for refugee team runner James Chiengjiek.

The 29-year-old who fled South Sudan aged 13 to avoid becoming a child soldier tumbled to the  track after the runners came in from their lanes.

He ran on to finish but was inconsolable at the end, sinking to his knees and holding his head in his hands as his rivals walked off without saying a word or even a pat on the back.  

Brownlee wins mixed triathlon
Britain won the mixed triathlon relay at the Tokyo Olympics on Saturday as Jonny Brownlee finally earned a gold medal in his third Games.

The British team of Brownlee, triathlon individual silver medallist Alex Yee, Jessica Learmonth and Georgia Taylor-Brown, who also won silver in the women's event, finished 14 seconds clear of the US team, with France taking bronze.

It means Brownlee now has a full set of Olympic medals after a career spent chasing his elder brother Alistair.

He finished in the silver medal position behind him 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, after the inaugural Olympic mixed triathlon.

"A super, super amazing day and being part of the team and to win the first ever mixed team relay at the Olympic Games in triathlon, perfect day."

The 31-year-old said he was unlikely to feature at the 2024 Paris Olympics.

"I'm definitely less sure now after my run in that relay because I didn’t think I had that speed in me. But no, I'm pretty confident that'll be my last Olympic Games now. I'm ready for a different challenge now and we'll wait and see what the future holds."

Dressel triumphs in 100m butterfly gold medal
American powerhouse Caeleb Dressel smashed his own world record to win the men's Olympic 100m butterly title Saturday for his third gold medal of the Tokyo Games.

The two-time world champion touched in 49.45 seconds to better his own world best 49.50 set at the 2019 world championships.

Hungarian 200m fly champion Kristof Milak was second in 49.68 -- only the fourth man ever to go under 50 seconds -- and Noe Ponti of Switzerland third in 50.74.

Dressel was always the man to beat, having set the third-fastest time ever in the heats  with no other active swimmer before the final close to his world record.

It was the fourth new world record in the Tokyo pool, and the second individual one after South Africa’s Tatjana Schoenmaker crushed the long-standing 200m breaststroke mark on Friday.

Australia's women set a new benchmark in the 4x100m relay, as did China in the women's 4x200m relay.

Dressel remains on track for possible six gold medals, having already won the 100m freestyle crown and helped the US team clinch the 4x100m relay title.

He is expected to later Saturday contest the 4x100m mixed medley relay final and is the undisputed favourite in the 50m freestyle final on Sunday.

Dressel is also expected to swim the 4x100m men's medley relay on Sunday, the final event in the Tokyo pool.