US great Michael Phelps closed his Olympic individual career in breathtaking style as America ruled the pool and records tumbled on the cycle track.
Phelps' last-gasp surge won the 100m butterfly by a whisker from Chad le Clos and Evgeny Korotyshkin, taking his record tally to 21 Olympic medals, including 17 golds.
Irrepressible team-mate Missy Franklin won her third gold medal in a world record 200m backstroke and America's Katie Ledecky, 15, became the Games' youngest swim champion with a stunning 800m which threatened the world record.
The athletics got under way at the 80,000-capacity Olympic Stadium which was packed for the morning session and the evening when Tirunesh Dibaba of Ethiopia retained her 10,000m title.
Poland's Tomasz Majewski did the same in the men's shot put, but British heptathlete Jessica Ennis caught the crowd's attention with a dazzling start as she blazed through the first four events to lead the competition.
She set the fastest 100m hurdles time ever in a heptathlon -- her mark of 12.54sec would have won gold in the individual event in Beijing four years ago -- and followed that with a personal best 22.83sec in the 200m.
"I knew coming into this I was in good shape but to be honest I couldn't have imagined performing like this," she said. "I've definitely exceeded my expectations today."
Carmelita Jeter of the US threw down the gauntlet to the Jamaican sprint queens by running a scorching 10.83sec in the first round of the 100m -- that time would have earned her a silver medal in Beijing.
In tennis, Wimbledon champion Roger Federer won the longest three-set match in the Open era, 3-6, 7-6 (7/5), 19-17 against Juan Martin del Potro in four hours and 26 minutes, to reach his first Olympic final.
"I don't think I've ever played as long a set in a best-of-three match," Federer said of the marathon decider.
And Britain's Andy Murray beat world number two Novak Djokovic 7-5, 7-5 to set up a repeat of last month's Wimbledon final. Serena Williams will play Maria Sharapova in the women's final on Saturday.
Anna Watkins and Katherine Grainger won the women's double sculls rowing to set off Britain's second three-gold haul in two days.
In the Velodrome, Britain smashed the world record to win the men's team pursuit and Victoria Pendleton took the women's keirin with a last-lap surge.
Britain's track cycling dominance was underlined when the women set a new world mark in qualifying for the team pursuit.
Olympic swimming will bid farewell to Phelps on Saturday, when he races the 4x100m medley, and he made his last individual event one of his very best.
From seventh at the turn in the 100m butterfly, Phelps surged to snatch gold with a performance which left his watching mother shaking her head in amazement.
The nail-biter meant Phelps, who on Thursday became the first man to win an individual Olympic swimming title three times in a row, achieved the same feat on consecutive nights.
"This was a bigger margin of victory than the last two combined, so we can smile and be happy," Phelps said. "It was fun."
Franklin, the 17-year-old American, showed where swimming's future lies as she won the 200m backstroke in a world record 2min 04.06sec.
History was made in the judo competition where Wojdan Shaherkani became the first woman from Saudi Arabia to compete at an Olympics.
Shaherkani, 16, lasted a mere 82 seconds after a build-up which had been overshadowed by a row concerning her hijab.
"I was disturbed and afraid at the beginning, it was my first time in a big competition and there was a lot of pressure because of the hijab issue," she said.
She was not the only teenage trail-blazer on a day of firsts for Muslim women -- coincidentally on a Friday, the religion's day of prayer, and during Ramadan, its month of fasting and devotion.
At the Olympic Stadium, Maziah Mahusin, 19, became Brunei's first female Olympian and Noor Hussain Al-Malki, 17, broke new ground as Qatar's first woman track athlete at the Games, before pulling up injured in the 100m.
"I think it is a great symbol, it is a great message in those countries and I think we're entirely happy about that," said International Olympic Committee spokesman Mark Adams.
"Did we expect them to win gold medals? Probably not. But they are here, they are competing and I think we should be very happy."