Ennis has struggled with her run-up this season, committing seven fouls in 12 attempts during two competitions earlier this summer, and managed just 5.95m with her first attempt.
However, the 26-year-old then hit the board perfectly on her second to soar out to 6.40m, just 11cm down on her personal best, and there was better still to come in the final round as another capacity 80,000 crowd roared the former world champion down the runway and out to 6.48m for 1,001 points.
World champion Tatyana Chernova was the only athlete to jump further than Ennis, a leap of 6.54m moving her up from ninth to third and 290 points behind Ennis, but reigning Olympic champion Nataliya Dobrynska's challenge is over after two fouls and a last aborted attempt of 3.70m dropped her to 33rd.
There was also drama on the track as South Africa's Oscar Pistorius became the first double amputee to compete in the Olympics and Paralympics, the 'Blade Runner' making the semi-finals of the 400m as controversial defending champion LaShawn Merritt crashed out.
Pistorius was second in the opening heat to advance, while Merritt pulled up injured around 120m into heat six after failing to recover from a hamstring injury suffered in Monaco recently.
Merritt said: "You couldn't even imagine [what I've been through to get here]. Countless hours of treatment from 8am till about 10pm.
"I thought I could come out and get my way through but I'm still young and I have a long career ahead of me. There was no reason to push it. Once I got on that back stretch I started floating and I felt it."
Merritt tested positive three times in late 2009 and early 2010 but eventually proved to the satisfaction of authorities that he had ingested the steroid derivative DHEA accidentally through use of a so-called male-enhancement product.
He served a 21-month suspension but remained ineligible for the Olympics because of an IOC rule banning athletes with doping sentences longer than six months, until CAS ruled that the IOC ban unfairly penalised athletes twice for the same offence - a ruling which paved the way for the British Olympic Association's own drugs bylaw to be overturned.
British trio Nigel Levine, Conrad Williams and Martyn Rooney joined Pistorius in the semi-finals, along with Grenada's world champion Kirani James and Belgium's Jonathan Borlee, who demonstrated how quick the track is with a national record of 44.43s.
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