Jessica Ennis dropped to second and is set to miss out on World Championship gold after managing just 39.95 metres in Tuesday morning's javelin.
The Briton will need to beat Russia's Tatyana Chernova by at least nine seconds in the heptathlon's concluding 800m at 1200 BST.
With Chernova's personal best over two laps a full two seconds faster, Ennis may have to settle for silver.
Ennis is attempting to become the first British woman to retain a World title.
With a tally of 5887, Chernova's cushion over second-placed Ennis is a huge 133 points, with Germany's Jennifer Oeser a further 143 back in third.
Although the reigning European and World champion had led by 151 points coming into the second day, projections based on each athlete's relative strengths in the remaining three events had Chernova moving ahead.
But while the Russian opened the day with 6.61m in the long jump, Ennis responded with a leap of 6.51m in the second round to match her personal best.
It meant that the Briton maintained a lead of 118 points with five events left, a far better scenario than Chernova's PB of 6.82m had suggested.
But when Chernova went out to a season's best of 52.95m in the second round with Ennis unable to get anywhere close to her personal best of 46.71m, the gold medal began to slip from the 25-year-old's grasp.
European indoor 3,000m champion Helen Clitheroe made it through to the final of the 5,000m as a fastest loser after coming home eighth in her heat in 15 mins 37.73 secs, while 39-year-old Yamile Aldama reached the triple jump final after a fine season's best of 14.35m.
The Cuban-born Aldama competed for Sudan at the 2004 Olympics but was cleared to compete for Britain earlier this summer, 10 years after first applying for British citizenship.
She said: "My age is not a problem. I proved to myself I still can do it, I'm still in good shape and it's going to go well in the final. Wednesday's my youngest son's first birthday so I'll look to give him a good present."
Tom Parsons and Martyn Bernard went out of high jump qualifying, Parsons clearing 2.25m at his second attempt but failing at 2.28m and Bernard crashing out with three failures at 2.21m.
Parsons said: "I felt pretty good warming up, but it was a bit hit and miss.
"It's probably the best qualification standard in the history of high jump but still I should be able to jump those sort of heights as I felt good - but I just didn't pull it out."
And James Shane, winner of the national 1500m title last month, failed to progress after missing out on a fastest loser's place by an agonising single place and nine one-hundredths of a second.