The Canberra rider couldn't contain her emotions after a "nightmare" BMX final in London.

Instead of standing on top of the podium with a gold medal around her neck, the 21-year-old was trying to figure out where she went wrong as she crossed the line fifth.

Nothing could ease the heartbreak. After the heat of battle and with her emotions spilling over, a tearful Buchanan was inconsolable.

"You know what," Buchanan said before pausing to wipe away some tears. "It seriously feels like a nightmare."

"My house has burnt down, my brother has broken his neck, I've had setbacks in my life before but nothing compares to this."

As she braved the post-race media, it seemed as if the pain would never disappear.

In time it will pass as she regroups and, well, gets back on her bike.

A decade ago Buchanan's brother Sam broke his neck in a BMX race. The easy option then would have been to end her love of the adrenaline junkie sport.

In 2003 her family home in Canberra's south was destroyed by the bushfires which devastated the capital.

At the end of last year one of her closest friends, Dane Searls, died on the Gold Coast.

She's had injuries, been written off by critics and sacrificed a "normal life" to chase her BMX goals around the world.

But every time something has gone wrong, she has bounced back.

And as hard as it was to be hit with the realisation she didn't finish on the podium, there's already a fire burning in her stomach for the Rio Games in 2016.

"Right now it feels like a bit of a nightmare, I just want to get out of here. I've been riding great all day, this is the best I've ever rode in my life.

"I prepared well, I've done every single little one percenter that I could probably do leading into this race.

"I knew I had everything in my favour and I don't know if it was the pressure or what it was, but I missed the gate on an Olympic stage.

"I got fifth, I'm happy. But at the end of the day, I wanted gold."

It was hoped Buchanan and Sam Willoughby would take double gold for Australia. But while Willoughby won silver, Buchanan was left shattered.

Buchanan had been one of the best riders all day. Earlier in the week she had qualified fastest for the semi-finals in a solo race. But everything changes when there are eight riders jostling for position.

Despite the pressure of chasing her dream, Buchanan navigated the semi-finals with relative ease.

She booked her spot in the gold-medal race and she was poised to achieve what she had been chasing since the sport was added to the Olympic program.

Four years ago Buchanan was too young to compete in Beijing, depsite being one of the best riders in Australia.

Every day since she's worn the Olympic rings on a necklace given to her by parents Laurence and Gail.

London was supposed to be the stage she used to launch herself to the top of the sport.

She is No2 in the world and the time trial world champion. But when it came to the gold-medal race, she missed the start, fell behind and never recovered.

To describe it as heartbreaking is an understatement.

"I truly believe that it's all of the hard times that make you step up to the next level and that's what makes you a champion.

"I'm going to hold my head high and I've had a lot of setbacks in my life and a lot of hard races with things not going my way.

"I have no idea [what to do now]. This is not what I planned. I wanted a gold medal.

"I've got my whole family here for support but I guess I get on that plane home soon and start preparing for four years time in Rio."

There was drama and concern early in the day when Brazilian rider Squel Stein had an ugly crash on the second jump of her semi-final.

Stein was stretchered off the track and taken to hospital with her coaches yet to comment on her injuries.