Ennis recovers from slow start to set up a nail-biting finish

Having recovered from what she confessed was "a shaky start" in the opening two events, Great Britain's Jessica Ennis leads the heptathlon this morning. Her first-day tally of 4,078 points – after four of the seven events – leaves her just two down on her running total en route to the European title and a lifetime best tally of 6,821 points in Barcelona last summer, but the title is by no means a certainty.

That's because, in Tatyana Chernova, she has a serious rival. The Russian, who lies in second, some 151 points back, is strong in all three of the second-day events – long jump, javelin and 800m. Indeed, if you project forward the scores from the best performances the two rivals have achieved in those events over the past 18 months, Ennis would emerge the winner by just three points. That margin would equate to 0.20sec in the deciding event, the 800m.

"It's so tight," Ennis said. "It's going to be tough tomorrow. These girls have got strong second days, so I have to make sure I am completely on it."

Chernova's emergence is not a surprise. When Carolina Kluft abdicated her throne as the unbeaten queen of the heptathlon, after winning a third world title in Osaka in 2007, it was expected that the prodigiously gifted Chernova would be the athlete to assume the Swede's crown.

Third place in the Beijing Olympics, eighth in the 2009 World Championships and fourth in the European Championships last year have not matched the expectation loaded on the young Russian, who won the world junior title in 2006 in commanding fashion – and who comes from Olympic gold medal-winning stock: her mother, Lyudmila, ran the third leg for the victorious Soviet 4 x400m relay team at the Moscow Olympics in 1980.

Yet now, at 23, the towering Chernova is threatening to finally fulfil that rich promise and cast a shadow over Ennis's historic World Championship mission in South Korea.

The 25-year-old Sheffield woman has not lost a heptathlon since 2007 and has a proven champion's pedigree. However, as she seeks to retain the global crown she lifted in Berlin two years ago, and become the first British athlete to win back-to-back World Championship titles, Ennis knows she faces the prospect of a tooth-and-nail battle with Chernova.

One key factor in favour of the woman from the Steel City is that she has already shown her formidable mettle after getting off to an uncharacteristically wobbly start. Her time in the 100m hurdles, 12.94sec, was by no means a disaster but clipping two hurdles and finishing 0.01sec behind the American Hyleas Fountain was a disappointment by Ennis's standards.

Clearing only 1.86m in the high jump – 9cm down on her British record height – was certainly a setback, leaving Ennis 41 points behind Fountain, the Olympic silver medallist. Still, the Briton then uncorked a lifetime best of 14.67m in the shot and finished the day with a 200m victory in 23.27sec.

"It's been a stressful day," Ennis said. "I was frustrated after the first two events, but I'm happy to be in the lead now."

Chernova had reason enough to be happy with her day, too, having clocked a personal best in the 100m hurdles (13.32sec), cleared a season's best in the high jump (1.83m), thrown a year's best in the shot (14.17m), and recorded a lifetime best in the 200m (23.50sec). If she maintains that form in her stronger events today, it could all come down to an 800m sprint finish.