Adlington blown out of the water by superb 15-year-old

Briton's worst fears realised in 800m final where she settles for bronze as unheralded American leads from start to finish

It was Rebecca Adlington who had urged caution all along. In the countdown to London, first through her sparkling performance here in the Olympic trials and on through a training programme that smoothed a path towards the podium, Adlington herself sought to check rising expectations. Watch out, she said, someone could come from nowhere and win, someone could do what she had done in Beijing four years ago.

Adlington was proved right last night, not that that will offer any consolation to add to her second bronze of London 2012. Katie Ledecky did not quite come from nowhere but a 15-year-old had no right to produce a performance of the magnitude the American did last night. It was the most outrageous swim of the Games.

Only Adlington has ever swum a faster 800m freestyle, the toughest event in the women's programme, and that was to win in Beijing. Ledecky had emerged from nowhere at the US trials in Omaha in June, surprising even her own coach – she was supposed to be targeting Rio 2016 – to set the second fastest time of the year. Only Adlington, in the British trials in March, had swum quicker.

The Dream Team, the US basketballers, were in the Aquatics Centre audience last night on what was supposed to be the home team's big night. Instead it was a night of American dreams – as the precocious Ledecky followed Michael Phelps, Missy Franklin, another of the teenagers to have lit up the pool this week, to the top of the podium. As Phelps returned through the call room after collecting his medal he high-fived his compatriot. "Enjoy yourself," he said. That, said Ledecky, inspired her.

"It was such a difficult race, she swam so fast," said Adlington. "She swam absolutely incredible. Fifteen – it's amazing. The 800 was my event. I think the pressure and the expectation going into this meet has been a little bit of a battle.

"I would have liked the time to have been quicker. I've done that time all year and I don't know what happened. Everything just kind of caught up with me. I gave it my absolute all and I'm sorry that I didn't get the gold for everyone that was expecting me to.

"But I am so proud to get a bronze – it's nothing ever to be embarrassed about. I hate it when people say that silver or bronze is losing because you have not done my sport. Swimming is one of the hardest sports to medal at. It is so, so difficult and I hope the public realise this and will be proud of me for getting that bronze."

Adlington admitted she was disappointed with her time last night. It marks a crushing end to an Olympic cycle that has had more ups and downs than a mountain stage of the Tour de France. The 2009 world championships in Rome was a low point, a bronze in the 400m was fair enough but fourth in the 800m was not. A year later the European championships ended with Adlington in tears and to get here ready to compete again has taken, according to her coach Bill Furniss, a great deal of "blood sweat and tears".

And it ended in tears too. Adlington will take a break from the sport until October – she is to train for a charity bicycle ride in Africa – after that she will sit down with Furniss and decide whether she will remain in the sport. "It's been a hell of a four years," she said. "It's been an emotional four years."

Swimming in the same lane four that minutes earlier had hosted Phelps collecting a 17th gold, the race was scripted to be about Adlington seeing off the challenge of her great friend and even greater rival Lotte Friis to become the first British woman to win three Olympic gold medals.

Adlington has been racing Friis, who is a year older, since she was 15. At last year's world championships the Dane and the Briton had matched each other stroke for stroke until Adlington snatched it. That announced her return to her best. Ledecky, meanwhile, was at school, Stone Ridge School of the Sacred Heart in Bethesda, Maryland.

Adlington is known for paying little heed to what her rivals are doing, but Furniss will have noted Ledecky's time in the US trials. She was quick in her heat on Thursday too but faded after seemingly going out too quickly.

Ledecky, who only turned 15 in March, went out fast again last night. It looked the flying start of the naïf. Behind her Friis and Adlington took up their duel. But Ledecky kept going, and slowly it began to dawn on the vocal home audience that she was not going to fade.

Friis broke first. Adlington ploughed on but the gap refused to close. Ledecky led from start to finish, her split time consistently hitting the 30-31sec mark. And just to rub salt in Adlington's raw wounds Mireia Belmonte Garcia, a 21-year-old butterfly silver medallist but a distance novice, passed her over the final 100m.

Ledecky, every inch the gawky teenager, appeared as astonished as anybody as she flew home in 8min 14.63sec, just half a second slower than Adlington's world record. The Briton came home in 8.20.32, over five and a half seconds later. It can never have been more painful to have been proved right.

Kids rule in the pool: Swimming successes

Katie Ledecky, 15

The American conquered Adlington in the 800m freestyle last night.

Ruta Meilutyte, 15

Won 100m breaststroke. Born in Lithuania, but lives in Plymouth, and goes to the same school Tom Daley attended.

Ye Shiwen, 16

The Chinese teenager surged to golds in 200m and 400m medleys.

Missy Franklin, 17

Nicknamed "the missile", the American has won four medals at this Olympics, including three golds.