Lawrence Okoye proved his medal credentials with an impressive final throw in discus qualifying in the Olympic Stadium this morning.
The 20-year-old produced a throw of 65.28 metres with his third attempt, the fourth furthest of the morning, to go through to the final automatically.
The former rugby player, who has deferred a place at Oxford to study law to concentrate on athletics, is appearing in his first major championship in his home town.
He improved his British record to 68.24m in May and his qualifying throw was little more than a metre down on Estonia's Gerd Kanter's longest effort.
Okoye knew his final throw was a big one, celebrating before the discus even landed, pumping his fists and roaring with delight before running over to his coach John Hillier.
"It was unbelievable, really tough. The standard this year is much higher than it's ever been," he said.
"It took a lot to get through and I'm glad to get the job done.
"Tomorrow I have a bigger job to do with the big guys coming through.
"It's going to be tough to get amongst them but I am ready to do it."
Okoye admitted his emotion-fuelled reaction was a mixture of relief and delight.
"The two years I've spent doing this would have gone down the pan if I didn't get through," he added.
"The first (throw) was so bad, but I came back and showed some character which is good. I will be ready come tomorrow."
Brett Morse and Abdul Bukhari failed to make it through to the final, the former registering only one legal throw of 58.18m and the latter getting a best of 60.08m.
Morse said: "In the first round I walked in and wound up and the crowd just went mental so I was a little nervous on that throw. I didn't actually hit any of the throws properly."
Bukhari said: "I have to go away and think about preparing myself technically. My warm-ups were awesome but I think I tightened up in my three throws."
Great Britain's 1500m runners earlier rescued what was threatening to become a disappointing morning for the home nation.
Hannah England and Lisa Dobriskey, who have struggled badly with fitness and health issues in the build-up to the Games, showed they are coming into form at just the right time with impressive runs to make the semi-finals.
World Championship silver medallist England has been hampered by a spiked Achilles tendon she suffered in a race in May that kept her out for around six weeks, but she always looked well-placed and in control as she came home fifth in four minutes 05.73 seconds.
Months after being diagnosed with potentially fatal blood clots on her lungs and told to give up on the Olympics, Dobriskey showed plenty of strength on the final lap to win a slow heat in 4mins 13.32secs.
Laura Weightman, who is trained by former world record holder Steve Cram, finished sixth in the third heat 4m 07.29s to also go through.
England declared herself "really pleased" with her run.
She said: "I felt like myself, I felt like I had presence in the race.
"I felt like that in the first round (at the World Championships) in Daegu, like I really belonged, and it's nice to feel like that again after the time I've had out.
I have been feeling great in training the last few weeks and it's important to transfer that into races and I really felt like I managed that today."
Dobriskey, who admitted recovering in time to make the Games has been her greatest achievement, said: "I felt strong and controlled and really powerful which is like my old self again.
"To be able to cope with a slow race was my biggest fear so I am really happy to have overcome that."
Weightman added: "I tried to stay calm and work my way through the field. I knew they would come back to me and the crowd in the last 200m really got me through the line."
Andrew Osagie, the world indoor 800m bronze medallist, lived very dangerously as he claimed the third place needed to progress in his race by 0.03s.
In a race controlled by world record holder and gold medal favourite David Rudisha, Osagie left himself plenty of work to do in the home straight, just managing to come through before the line to finish in 1:46.42.
Asked if he was breathing a sign of relief, Osagie said: "A little bit. The heats are always nervous. I am renowned for running rubbish in the morning."
Gareth Warburton, who won selection on appeal after initially being left out of the Olympic team, went out, finishing fifth in his heat in 1:46.97.
There was also disappointment for Michael Rimmer, who completely ran out of steam in the final 50m of his heat to finish fifth in 1:49.05.
Rimmer, who has struggled with injury since winning European Championship silver two years ago, said: "I have no answer to be honest, shocked.
"My legs did not have the change of gears. I am really confused because I was running PBs in Portugal (at the pre-Games training camp)."
Warburton, who was competing in his first major championships, said: "I take experience from this. I have learned a lot, not just from running here today but from the whole run-up."
Tiffany Porter qualified for the semi-finals of the 100m hurdles, but did little to ease fears she is not fully fit.
Porter, who was third in a very fast heat in 12.79, always looked sure of the top-three finish needed for progression, but appeared to be running close to flat out.
The 24-year-old was an injury doubt coming into the Games after she left the track at last month's Diamond League meeting at Crystal Palace in tears, a back injury having flared up.
United States-born Porter is one of the athletes in the team who has been dubbed a "Plastic Brit" by critics, but not by the vast majority of the crowd judging by the roar which greeted her introduction.
On the back injury, she said: "It's fine, I'm very confident and I will go out there and do my very best."
She lost her British record to Jessica Ennis in her stunning first event of the heptathlon on Friday.
The newly-crowned Olympic champion did not line up in the heats, having stuck with her decision not to compete in the hurdles.