Swimming legend Michael Phelps suffered a nervy start in his bid to become the greatest Olympian in history on Saturday while Chinese shooter Yi Siling claimed the first gold of the Games.
Phelps, the 27-year-old swimming legend, who has 16 Olympic medals, needs just three medals to surpass Soviet gymnast Larisa Latynina's record of 18.
But the defending champion just scraped into Saturday's 400m individual medley final by the skin of his teeth.
Phelps finished in 4min 13.33sec to grab the eighth and last spot in the final which takes place later Saturday, but sounded a defiant note afterwards despite his brush with disaster.
"A final spot is a final spot," Phelps said, while acknowledging he had been caught cold. "I didn't expect those guys to go that fast. I just wanted to try to get some good underwater, try to get some good times.
"You can't win the gold medal from the morning," he added.
American rival Ryan Lochte, who has established himself as a threat to Phelps in both the 200m and 400m medleys, was content to finish second in his heat behind South African Chad le Clos.
Le Clos's 4:12.24 was second-fastest of the morning, and Lochte was third-fastest of the day in 4:12.35.
In a major shock at the Aquatics Centre, reigning Olympic champion Park Tae-Hwan of South Korea was disqualified in the 400m freestyle for a false start.
Park touched first in his heat, but left the pool stunned as the disqualification flashed on the scoreboard. China's Sun Yang led the way into the final.
Chinese world number one shooter Yi Siling had the honour of claiming the first of the Games' 302 golds at the Royal Artillery Barracks in the women's 10m Air Rifle.
Yi defeated Poland's Sylwia Bogacka, with Yu Dan of China taking bronze.
"It's very exciting. Very happy. I'm very grateful to China. And to my mother and father who I love very much," Yi said.
Defending champion Pang Wei could make it a China double in the men's event later Saturday.
But Saturday also witnessed the first failed drugs test of the Games after Albanian weightlifter Hysen Pulaku was suspended for taking the banned steroid stanozolol -- the same drug that cost Canada's Ben Johnson his 100m athletics gold at the 1988 Seoul Games.
The International Olympic Committee (IOC) said the case showed drug cheats will be found out.
"Of course, it is always a sad day when a cheating athlete is caught. We hope there will be no more, but the message is very clear: if you are doping we are going to catch you," said IOC spokesman Mark Adams.
Just six days after playing a role in Bradley Wiggins's historic Tour de France yellow jersey triumph, Mark Cavendish was the favourite for the 250km men's cycling road race.
Cavendish, the world champion, was supported by David Millar, Wiggins, Chris Froome and Ian Stannard, the national champion, in a race set to finish in front of Buckingham Palace.
Meanwhile Roger Federer returns to Wimbledon, where he captured a record-equalling seventh title earlier this month.
The world number one begins his campaign to add singles gold to the doubles title he captured in Beijing in 2008 with a first round tie against Colombia's Alejandro Falla.
Elsewhere, legally blind archer Im Dong-Hyun, who set the first world record of the Olympics on Friday, and then added another as South Korea broke the team record, targets gold.
There are also golds on offer in the women's weightlifting and men and women's judo.
Meanwhile Britain's newspapers lavished praise on Friday's spectacular four-hour long ceremony, watched by an estimated one billion TV viewersI around the world.
The Times ran the headline "A Flying Start" on a souvenir wraparound photograph of the Red Arrows display team flying over the stadium while Rupert Murdoch's market-leading tabloid The Sun went simply with "Golden Wonder".
A budget of £9.3 billion ($14.5 billion, 12 billion euros) has been spent on bringing the Games back to London for a record third time.