US great Michael Phelps closed his Olympic individual career in breathtaking style on Friday 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 the 200m backstroke and America's Katie Ledecky, 15, became the Games' youngest swim champion with a stunning 800m which threatened the world record.
As athletics got under way at a packed, 80,000-capacity Olympic Stadium, poster-girl Jessica Ennis twice had fans on their feet with personal bests in the 100m hurdles and 200m. After four events, Ennis was top of the standings.
"Amazing! It's so loud," Ennis said of the crowd. "When you step up to jump or get on your blocks they just really get behind you."
Carmelita Jeter of the US ran a scorching 10.83sec in the first round of the women's 100m, a time that 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.
In early action, Anna Watkins and Katherine Grainger won in the women's double sculls rowing set off Britain's second three-gold haul in two days.
As attention switched to 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 also set a new world mark in qualifying for the women's team pursuit. After four events on the track, the hosts have won three and Germany one.
Olympic swimming will bid farewell to Phelps on Saturday, when he races the 4x100m medley, and he made his last individual event on Friday one of his very best.
From seventh at the turn in the 100m butterfly, Phelps surged to snatch it with a long glide at the death in a performance which left his watching mother shaking her head in amazement.
The nail-biter recalled Phelps' thrilling victory by one-hundredth of a second over Milorad Cavic in Beijing, as well as his triumph by four-hundredths of a second over compatriot Ian Crocker in Athens.
And it 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."
History was also 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."