Britain's Jessica Ennis claimed a 184-point lead in the heptathlon thanks to a sensational performance in front of a massive crowd in the Olympic Stadium.
Ennis finishes the day at the top of the heptathlon table with 4158 points. And fellow Brit, Katarina Johnson Thompson is in 14th place with 3769 points. Second to Ennis is Lithuania's Austra Skujyte with 3974 points.
Only a handful of seats in the 80,000-capacity stadium were empty when Ennis broke the British record and smashed her personal best with an exhilarating run of 12.54 seconds in the 100m hurdles. It was a time which gave American Dawn Harper Olympic gold in the individual event in Beijing.
The 26-year-old took 0.25secs off her PB and 0.02s off the previous national record set by hurdles specialist Tiffany Porter, while it was also the fastest ever hurdles time recorded in a heptathlon.
"I kind of felt strangely calm beforehand," Ennis said. "I'm normally quite nervous before the hurdles but I was quite calm and then coming out in the stadium and seeing everyone and the crowd, it's just such an amazing feeling. It gives you goosebumps.
"I am still so shocked at the hurdles time. I can't believe it to be honest, I really can't. I knew I was in good shape, hurdles have been going well and if I had run 12.80 or 12.70-something I would have been over the moon, but 12.54, I literally can't believe it. It's so crazy."
Ennis set another personal best of 22.83secs in the 200m this evening for a total of 4,158 points, her best ever first-day score and enough for a lead of 184 points over Skujyte.
Ennis was actually entered into the individual event - which starts on Monday morning - as a back-up if anything went wrong in the heptathlon, but remained uncertain as to whether she would double up.
"I had done all the hard work and training and it has been going well, but I think when you're in this environment with the crowd and the nervous energy, it just all comes into one, helps you perform and just pushes you on," she said.
"I'm a little disappointed with the high jump, I would have loved to get 1.89m, but it was roughly what I jumped in Gotzis where I set my personal best so I am kind of round about the same mark."
Johnson-Thompson admitted a faulty start in her hurdles heat had done her a favour after being taken aback by the rapturous reception from the crowd.
"It was surreal. I was told I said the word 'Wow', but I didn't realise I had done it," she said. "I am really happy we had a little faulty start as it took me time to just reassess myself and focus on the race instead of being overwhelmed by the crowd.
"I am glad I've got seven events because I think I will settle into it. I think I've settled into it already. The support is incredible. I am just so happy to be here. It's a dream come true really."
Told she was in bronze medal position, she joked: "That will soon change after the shot, don't worry!"
The long jump, javelin and 800m take place tomorrow.
There was more good news for Britain in the women's triple jump, with 39-year-old Yamile Aldama reaching the final with her first jump in qualifying of 14.45m.
Aldama, who turns 40 on August 14, previously competed for her native Cuba and Sudan and revealed that her 71-year-old mother Modesta has left Cuba for the first time in her life to come and watch the Olympic final.
Aldama, who had heavy strapping on the right shoulder she injured in Rome on May 31, said: "I have no words to describe the crowd. I've been to five Olympic Games and the qualifying is never like this."
Defending Olympic champion Christine Ohuruogu, Shana Cox and Lee McConnell all advanced to the semi-finals of the 400m, while world champion Dai Greene, training partner Jack Green and European champion Rhys Williams eased into the next round of the 400m hurdles.
Greene qualified fourth fastest overall, with world number one Javier Culson of Puerto Rico quickest in 48.33, and said: "It was relatively easy. I struggle to get up for the heats usually so I just had to make sure I qualified as winner of my heat."
Britain's Alex Smith also made the final of the hammer, but thought he was destined to miss out after finishing only 10th in the first qualifying pool.
"It was just such a brilliant experience being out there, though I could have done better," Smith said. "But it was my first major championships and I am proud of what I've done. I went over to speak to my coach and I couldn't hear what he was saying because everyone was cheering me."
Only one man threw further than Smith in the second pool, meaning his best throw of 74.71m took him into the final as the 11th of 12 qualifiers.
There was disappointment for Carl Myerscough and Stuart Stokes in the shot and 3,000m steeplechase as they bowed out of their respective competitions.
Myerscough, cleared to compete in London when the British Olympic Association's lifetime ban on drugs cheats was overturned, was 29th in qualifying with a best of 18.95m.
He said: "I felt wonderful out there, then each throw just something slightly went wrong each time. I just didn't get one together, it's very frustrating. I am massively disappointed because I was in great shape. It's just a huge disappointment right now."
Stokes was 12th in his heat of the steeplechase in 8:43.04, and said: "It was tough going, but the crowd was absolutely superb. I was on for a PB with five laps to go and the guys were just running away from me.
"But I've got no complaints, no regrets. I am going to walk off with my head held high. It's a great climax to a lot of hard work, it's probably going to be the last time I put on my spikes. I am 35, have a wife, two kids and am a full-time teacher."
Jessica's route to glory...
Long jump: 10.05am
Ennis equalled her personal best in Götzis, with a 6.51m effort, but she has struggled to hit the board this summer and will have only three attempts to get it right tomorrow. The most important muscle here is the calf in the take-off leg. 6
Having messed up in the javelin at the World Championships in Daegu last year, Ennis has worked long and hard on her technique this winter. She has found a consistent groove, throwing a lifetime best of 47.11m in Götzis. The shoulder and the triceps muscle are crucial to delivery. 6
Ennis has a best of 2min 07.81sec but is capable of going faster if need be – which she might have to if she is to beat Tatyana Chernova and Nataliya Dobrynska. The leg muscles are the most important – quadriceps, hamstrings and calves. 7
... and her rivals
The 24-year-old Russian is reigning world champion and has steadily improved since winning Olympic bronze in 2008. Chernova edges Ennis in the 800m – the definitive final event – and is significantly stronger in the javelin and long jump, where she will outclass most of the field.
The powerful Ukrainian, 30, won Olympic gold in Beijing and has enjoyed something of a resurgence this year after a disappointing 2011. She traditionally excels in the throwing events but is behind Ennis and Chernova in the track and field categories.
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