London 2012 gold medal hopeful Jessica Ennis lost her heptathlon world title to Russia's Tatyana Chernova in Daegu.
Ennis finished 129 points adrift after a poor javelin effort left her needing an exceptional 800m run to take gold.
The 25-year-old was less than a second clear of Chernova, with a nine-second margin required to snatch victory.
"I'm going to work hard at all the events and make sure I'm strong next year. I've evaluated it and I'm not too disappointed," said Britain's Ennis.
Her hopes of becoming the first British women to defend a world title were slim before the final event with Chernova's personal best over two laps a full two seconds faster than her own previous fastest time.
The Sheffield athlete duly launched an ambitious attack after a swift first lap and opened a lead of about 10 metres on Chernova before the pace took its toll in the home straight.
"I knew it would be very, very difficult. I was being realistic but I had to give it everything and leave everything on the track," she said.
Ennis had led the points standings before a disappointing 39.95m, 13m short of Chernova's best throw, in the penultimate event - the javelin.
"Javelin has been one of my weaker events but I've never performed that poorly," she told BBC Radio 5 live. "But it's not something I'm going to overanalyse."
She added: "I have been in a few heptathlons now and they have ended up going in very different ways - I was leading in Berlin from start to finish and now here.
"It has been a massive learning curve."
Ennis, who won gold at European championships in 2010, finished on 6751 points, 129 adrift of Chernova, with Jennifer Oeser of Germany taking bronze with 6,572 points.
There were positive signs for Ennis heading into next summer's Olympics with a personal best in the shot put as well as in the 800m.
Her points total was also 20 more than the score that proved enough for victory in Berlin two years ago.
But even her personal best of 6,823 would not have been enough to wrestle gold from 23-year-old Chernova.
Commonwealth champion Louise Hazel, competing in her first World Championships, finished 15th with 6,149 points, 17 short of her personal best.
Elsewhere European and Commonwealth champion Dai Greene won one of three 400m hurdles semi-finals in a time of 48.62 seconds.
The Welshman's time was only bettered by Puerto Rico's Javier Culson ahead of the final at 1330 BST on Thursday, but Greene's compatriots Jack Green and Nathan Woodward failed to clock a time good enough to join him.
In the women's 400m hurdles, European bronze medallist Perri Shakes-Drayton was 0.01 seconds away from a place in the final as a fastest loser, while team-mate Eilidh Child also missed out.
European indoor 3,000m champion Helen Clitheroe advanced to the final of the 5,000m as a fastest loser after coming home eighth in her heat, while Cuban-born 39-year-old Yamile Aldama, cleared to compete for Britain earlier this summer, reached the triple jump final.
In the women's 1,500m, Hannah England reached the final as a fastest loser after a gutsy finish down the home straight carried her to sixth in the second semi-final.
Brett Morse was 12th in the discus final, while Barbara Parker ended 14th in the 3,000m steeplechase.
Tom Parsons and Martyn Bernard went out of high jump qualifying, and James Shane failed to progress in the 1500m after missing out on a fastest loser's place by nine one-hundredths of a second.