Great Britain's Jessica Ennis has made a stuttering start to the defence of her world heptathlon title in Daegu.
Ennis - bidding to become the first Briton to defend a world title - was second in the 100m hurdles and her 1.86m high jump was below her best.
The Sheffield athlete stands second overall, with Olympic silver medallist Hyleas Fountain of the United States in fine form, 41 points ahead on 2,228.
"I'm a bit disappointed with the high jump," admitted Ennis.
"I would have liked another height.
"[Also] I clattered a couple of hurdles, which kind of knocked me off balance, but I'm fine. I'm used to hitting hurdles. I would have liked to run faster - but 12.94 is still a good time.
"I was coming into this knowing it was going to be a challenge and I still expect that. It's going to be a tough competition, but I'm definitely going to give it my all."
Ennis would have expected to take a lead after these early events and then consolidate her position but must now chase Fountain, who has returned to her form of three years ago.
"We'll have to win this a different way," conceded Ennis's coach Tony Minichiello on Monday.
Fountain handed Ennis her first heptathlon 110m hurdles defeat since the 2006 European Championships, although the two were clearly ahead of the field.
She then cleared 1.92m, while Ennis took two attempts at 1.80 and 1.83, and a nail-biting three at 1.86 before failing at 1.89.
Ennis's compatriot, the Commonwealth champion Louise Hazel, stood 16th with five events remaining.
Tatyana Chernova, the Russian who took Olympic bronze three years ago and whose strengths mean she is likely to narrow the gap on the second day of competition, was close behind in third.
And former Olympic champion Nataliyia Dobrynska of Ukraine sat fifth.
Next comes the shot put at 1105 BST, and the first day finishes at 1235 BST with the 200m.
Day two begins with the long jump and javelin, followed by the 800m in the evening session.
British 400m hurdlers Dai Greene, Jack Green and Nathan Woodward all qualified for the semi-finals during Monday's morning session, with Greene fastest overall in 48.52 seconds.
"I felt I didn't want to expend too much energy in the heats but I didn't think I was going to be that fast," said the Welshman.
"At the same time I felt very controlled out there.
"My preparation has been superb coming into the competition. I'm very confident and I feel on track to do myself justice."
Perri Shakes-Drayton was comfortable in qualifying for the semi-finals of the women's 400m hurdles and Eilidh Child also made it through.
Brett Morse was the 12th and final qualifier for the discus final but fellow Britons Carl Myerscough and Abdul Buhari were eliminated.