Britain, dominant in track cycling in Beijing four years ago, took its second and third gold medals in the velodrome, while New Zealand picked up two rowing golds in the space of 40 minutes.
Michael Phelps, the most decorated Olympian of all time, won his 21st medal by claiming the men's 100 metres butterfly.
Katie Ledecky, 15, took the women's 800m freestyle title and another teenage U.S. swimmer, 17-year-old Missy Franklin, grabbed her third gold medal of the Games in the 200m backstroke, breaking the world record in the process.
Older competitors also had their day.
Sergei Martynov, a 44-year-old Belarussian, used a 13-year-old gun and bullets from the Soviet era to win the men's 50 metres prone rifle with a world record score.
In archery, Oh Jin-hyek, 30, won South Korea's first men's Olympic individual archery gold medal after beating Takaharu Furukawa of Japan in the final at Lord's cricket ground.
"I was not as good at sports in my 20's, but now I've turned 30 and I want to keep doing sports for a long time," said Oh, who had tried to break into the mighty national Olympic team for a decade.
TRACK THRILLS
With the start of the athletics, the jewel in the Olympic crown, excitement began to build towards Jamaican Usain Bolt's defence of his 100 metres title on Sunday and his 200 crown four days later.
Ennis, the London Games poster girl whose every move was greeted with deafening roars at the 80,000-capacity stadium, was hoping to add to Britain's rising medal tally after setting a world best time for a heptathlete in the 100 metres hurdles.
Britain's former world champion followed up with a solid high jump but Lithuiania's Austra Skuyte set an Olympic record for the shot put in the heptathlon to take the overall lead.
A short but heavy morning downpour could not dampen spirits at the main arena, where loud recorded music from Coldplay to Queen blared out in a cross between an athletics meet and a pop concert.
World champion Vivian Cheruiyot of Kenya was beginning her bid for a distance double in a much-anticipated 10,000 metres final where Ethiopian Olympic champion Tirunesh Dibaba, who has the fastest time of the year, is expected to be her main rival.
Cheruiyot also has her eye on the 5,000 next week as she attempts to replicate her double gold from last year's world championships.
At Wimbledon, Roger Federer of Switzerland remained on course to repeat his heroics in the Grand Slam in July, beating Argentina's Juan Martin del Potro in a marathon 19-17 third and final set for a place in the final.
Federer will now face local hope Andy Murray, who rode a wave of British euphoria to beat Serbia's world number two Novak Djokovic 7-5 7-5 and set up a repeat of last month's Wimbledon final against the Swiss maestro.
New Zealand struck gold twice on the water, with Mahe Drysdale taking the men's single sculls and men's pair Eric Murray and Hamish Bond cruising to victory in comprehensive fashion.
Britain's Katherine Grainger and Anna Watkins triumphed before 25,000 ecstatic fans at Dorney Lake in the women's double sculls - a dream come true for Grainger after three previous silvers.
Germany powered to gold in the men's quad sculls, finally getting their revenge on the young Croatian crew who had beaten them all season.
But another German rower, Nadja Drygalla, who has already finished competing at the Games, voluntarily left the Olympic village following reports that her boyfriend was a far-right extremist.
In cycling, the British men beat Australia in the team pursuit, setting a world record, and Victoria Pendleton won the women's keirin, a day after being disqualified with Jessica Varnish in the team sprint.
Britain's men took cycling track team sprint gold on Thursday, but the taste of victory was soured when German-born rider Philip Hindes admitted to falling over on his bike on purpose in the heats to avoid being disqualified.
"I did it on purpose to get a restart ... it was all planned really," he told reporters, prompting shock among British media and leading to calls for a change in the rules.
It also raised uncomfortable questions about gamesmanship at the Olympics, after eight badminton players were thrown out for deliberately losing matches to manipulate the draw, breaking the spirit, but not the rules of their sport.
British cycling officials later said Hindes' comments were lost in translation and the International Olympic Committee has no plans to investigate the incident "at present".
China's head badminton coach Li Yongbo took a swipe at the sport's federation and said enough punishment had been dealt to the eight expelled players and their teams for their part in the match-throwing scandal.
Li was ordered to make a public apology by China's national delegation this week and said he was to blame for the negative tactics of his world champion pair Yu Yang and Wang Xiaoli, who were among those kicked out along with two Indonesians and four from South Korea.
"I think the punishment is already too excessive as it is. Really. It's already too much," Li told reporters on Friday.
"It only shows that our (badminton) federation is really not up to the mark."
