American runners are facing their worst Olympic Games in history after failing to win a single gold medal on the track at the Tokyo Games.

When the curtain came down on the World Championships in Doha two years ago, the United States looked set for a new era of dominance.

Christian Coleman had bagged the 100m gold, Noah Lyles had claimed an emphatic victory in the 200m, and the American team departed Qatar with an impressive haul of 14 gold medals.

But fast forward two years and the Americans are facing an inquest into what has gone wrong in Tokyo, where there is every chance the US men could fail to win an individual running gold medal for the first time in Olympic history.

While the US women have contributed two gold medals on the track, through 400m hurdler Sydney McLaughlin and teenage 800m star Athing Mu, the men have so far come away empty-handed.

Across the board, the US men's medal hopes have wilted in the fierce sun in the Olympic Stadium.

Trayvon Bromell arrived in Tokyo as heavy favourite to become the first American winner of the 100m gold medal since 2004 but failed to even reach the final.

World champion Lyles was similarly favoured to claim the 200m crown, but had to settle for bronze. 

- 'Total embarrassment' -
In the 110m hurdles, another world champion, Grant Holloway, was upset by Jamaica's Hansle Parchment, while in the 400m hurdles, Rai Benjamin was undone by the brilliance of Norway's Karsten Warholm.

In the 800m, an event won by the USA's Donavan Brazier in Doha, who failed to qualify for Tokyo, Clayton Murphy finished last. 

The dismal run continued in the 1,500m, where Matthew Centrowitz, the 2016 Olympic gold medallist, was eliminated in the semi-finals.

Arguably the nadir came on Thursday, when the men's 4x100m relay team were knocked out in the semi-finals after an error-strewn performance left them in sixth. 

It was the first time in Olympic history the US 4x100 had failed to qualify for the final from a completed heat.

US sprinting greats Carl Lewis and Michael Johnson were aghast at the relay display.

"It was a total embarrassment and completely unacceptable for a USA team..." Lewis wrote on Twitter, while Johnson later branded the showing "embarrassing and ridiculous."

US athletes themselves have been at a loss to explain what has gone wrong in Tokyo. 

Some wonder if the US decision to cancel a pre-Olympic training camp in Japan due to Covid-19 concerns undermined the team's preparations.

"We haven't been here that long, we got here on short notice," US 400m runner Michael Cherry said Thursday after a fourth place finish in Thursday's final. 

"Every other team had training camp, but that's no excuse. We are still expected to come out here and execute how we are supposed to. it's just not happening right now."

- Overblown hopes? -
Lack of practice was cited as a factor by sprinter Ronnie Baker after a poor baton handover between him and team-mate Fred Kerley had contributed to the US exit in the relay. 

"We are all running fast right now," Baker said. "Fred is running 9.8sec and I am running 9.8sec. Trying to time that up perfectly with a couple of practices is tough."

Others point to the challenge US athletes face in attempting to peak for two major championships -- the US Olympic trials and the Olympics -- within weeks of each other. 

In 2019, the US squad benefited from an interval of more than two months between the trials and world championships in Doha.

Even so, not everyone is convinced scheduling is an issue.

"It really depends on how you plan your season out," said 110m hurdler Devon Allen after finishing fourth in Thursday's final.

"To peak for four weeks, for five weeks, like we do for the trials and the Olympics isn't really that tough."

Other athletes suggested the US may have had overblown medal expectations for the Olympics.

"I think maybe you guys just took it for granted all those years," eliminated 1,500m champion Centrowitz told reporters on Thursday.

"Before 2012, no one was winning and I wouldn't even say a lot have been winning. I don't know what the expectation was. I know we had more medals in 2016, but that's just the way it goes sometimes."

Canada's Quinn to become first trans Olympic medallist
Canadian footballer Quinn will become the first openly transgender athlete to win an Olympic medal on Friday in another trailblazing moment at the Tokyo Games for the marginalised community.

Quinn -- who goes by a single name and uses the pronouns "they" and "their" -- will play in the women's gold-medal match against Sweden, guaranteeing a medal regardless of the result.

The 25-year-old has a long history with the Canadian team, debuting in 2014 and winning bronze at the 2016 Rio Games, but only came out as transgender last year.

"I wanted to be my authentic self in all spheres of my life and one of those is being in a public space," Quinn said at the time. 

"So that was one of the reasons behind it, because I was tired of being misgendered and everything like that."

The player's pioneering status at the Tokyo Games has until now largely been overshadowed by the presence of transgender New Zealand weightlifter Laurel Hubbard.

Hubbard, whom the International Olympic Committee acknowledges became the first openly trans woman to compete at the Olympics on Monday -- set off a firestorm of debate over her appearance.

Critics argued the New Zealander had physical advantages locked into her body from her developmental years as a male, making it unfair for her to compete against female-born lifters.

However, Hubbard's Games debut proved anti-climactic in a sporting sense when she failed to complete a lift.

The 43-year-old, who was twice the age of some of her rivals and had not competed internationally since before the coronavirus pandemic, later admitted she was "overwhelmed" to be in the spotlight. 

There are no questions about Quinn's sporting prowess -- the player is entering the prime years for a defensive midfielder and lines up at club level alongside top women's stars such as US star Megan Rapinoe.

- 'Fight isn't over' -
Quinn, who plays with the Seattle-based OL Reign in the US National Women's Soccer League, has also not faced questions about their presence on the Canadian women's team.

Athletes who transition from female do not attract the same scrutiny because they are not considered to have the inherent physical advantages of those born male.

"I am considered maybe one of the most digestible versions of what it means to be trans," the player told the club website.

"I'm white, I'm trans-masculine. I want my story to be told because when we have lots of trans visibility that's where we start making a movement and start making gains in society."

Like Hubbard, Quinn has spoken about the struggles of being transgender in a binary-focused world and being a role model at the Games for young people experiencing similar challenges.

"(I'm) getting messages from young people saying they've never seen a trans person in sports before," Quinn told public broadcaster CBC after Canada shocked tournament favourites USA 1-0 to make the final.

"Athletics is the most exciting part of my life.... If I can allow kids to play the sports they love, that's my legacy and that's what I'm here for."

Rookie Tomala wins 'crazy' walk gold
Poland's Dawid Tomala won an astonishing 50km walk Olympic gold on Friday despite only completing the distance once before as US sprint great Allyson Felix sets her sights on track and field history in Tokyo.

Tomala's feat in gruelling conditions set the stage for a big day of athletics, with Felix bidding to become track and field's most decorated female Olympian and Dutch runner Sifan Hassan pursuing an unprecedented distance treble.

Elsewhere on day 14, Sweden face Canada in the women's football final and six-time defending champions the USA take on Serbia in the women's basketball semi-finals.

With only three days of competition before Sunday's closing ceremony, China top the medals table on 34 golds, with the United States four behind and host nation Japan third on 22.

Tomala won Poland's fourth gold of the Games in punishing heat in Sapporo, the revised venue for the race walks and marathons intended to avoid Tokyo's high summer temperatures.

It was an incredible win by the 31-year-old, given he only completed his first 50km race walk this year in Slovakia, finishing fifth.

"This was only the second 50km in my life and I win it," he said. "It is crazy, right?"

Tomala crossed the line in 3 hours 50 minutes and 8 seconds, becoming the first Polish champion after three-time winner Robert Korzeniowski.

April Ross and Alexandra Klineman beat Australia's Mariafe Artacho del Solar and Taliqua Clancy to win the women's beach volleyball, securing USA's fourth title in the event.

- 'Fight to get here' -
Later Felix, 35, goes into Friday's 400m final level with Jamaican great Merlene Ottey on nine Olympic medals and looking to claim sole ownership of the record.

"It was a fight to get here. When I was younger, I never really thought about making a final," said Felix, who won silver in the event at the 2016 Rio Games.

"This time, you get older and it seems harder. You just have to get smarter and figure it out."

The Ethiopia-born Hassan, already the 5,000m gold-medallist, goes up against Kenya's defending champion Faith Kipyegon in the final of the 1500m, with the 10,000m still to come.

With two-time winner Mo Farah failing to make the British team, a new Olympic champion will be crowned in the men's 5,000m, where Joshua Cheptegei headlines a strong trio of Ugandans including Jacob Kiplimo and Oscar Chelimo.

Jamaica look runaway favourites in the women's 4x100m relay, led by double sprint champion Elaine Thompson-Herah and 100m silver medallist Shelly-Ann Fraser-Pryce.

In football, Sweden, the 2016 runners-up, take on first-time finalists Canada in the women's final in Yokohama.

The match, initially scheduled for 11:00 am, was relocated from the Olympic Stadium and pushed back to the evening because of heat concerns, with temperatures climbing to 32 degrees Celsius (89.6 Fahrenheit) on Friday.

The Netherlands will bid for a record fourth Olympic women's hockey title against Argentina after Britain beat India 4-3 to secure bronze.

World number one Nelly Korda held a four-stroke lead going into the third round of the women's golf tournament.

In karate's Olympic debut, three-time world champion Ryo Kiyuna of Japan is vying to become the first gold medallist in the men's kata, while China are favourites in the final of the men's team table tennis.

Away from the action, two Belarusian coaches have been kicked out of the Games over an alleged attempt to force a sprinter to fly home.

The International Olympic Committee said it had removed the accreditations of Artur Shimak and Yury Maisevich and they had left the Olympic Village.

The body said this week that it was investigating the pair over their role in the case of Krystsina Tsimanouskaya, who sought protection at a Tokyo airport to avoid being put on a plane home.