Ennis had a mixed day in her four events, recording personal bests in two – in the 100 meters hurdles and the 200m sprint – and below-par performances in the others, the high jump and shot.
But she still leads Lithuania's Austra Skujyte with 4158 points to 3974.
However, Team GB’s poster girl is adamant she’s taking nothing for granted as she bids to secure a gold medal in front of an adoring home crowd.
She said: “I’m so pleased to end the day with that, knowing it’s a fast track from the hurdles and running a a personal best in the 200m
“I’m still in shock about the hurdles but overall it's a little bit up and down.
"I’m a little bit disappointed with the high jump but it's not too bad.
"The shot was solid but still a bit disappointing but you can't complain to have two personal bests in a day.
“I’m going to have to have a really strong day on Sunday and make sure that everything is really solid and where it needs to be.
I’m glad to have got today out of the way and I’ve got a good score under my belt but there’s a lot more work to be done.”
Ennis finished the first day in the lead, thanks in part to a stunning performance in the 100m hurdles where she stormed home in 12.54 seconds – a record for a female heptathlete in the event, and a new British record.
After a decent, if unspectacular, high jump of 1.86m failed to do too much damage to her lead, Ennis entered the evening session with 2,249 points and a 25 point lead over USA’s Hyleas Fountain.
Another below-par performance in the shot put, where she managed 14.28m, half a metre blow her season’s best, relegated her to second behind Skujyte.
However, a personal best 200m run of 22.83 seconds saw the 26-year-old Olympic debutant regain her lead in fine fashion as she rounded off a good day with three events standing in her way of Olympic gold on Sunday.
Fellow Brits Katarina Johnson-Thompson and Louise Hazel spent the day in Ennis’s shadow and ended the day in 14th and 34th respectively.
Johnson-Thompson matched here total of two personal bests with her efforts in the 200m sprint, where she clocked 23.73 secs, and her high jump clearance of 1.89m – a centimetre above her previous best.
And the 19-year-old, rated as one of Team GB’s brightest heptathlon prospects, finished the day with an impressive 3,769 points as she continues to make her mark on the sport.
But Commonwealth champion Hazel suffered a disappointing start to her heptathlon campaign, finishing the day on a below par 3427 points.