The early casualties include Michael Phelps, winner of a record eight gold medals at the 2008 Beijing Games, who has a solitary silver medal after two of his seven races.
Saturday's highlight race at the Aquatics Centre pitted Phelps against Ryan Lochte, both 27, in the final of the men's 400 metres individual medley.
It proved no contest as Lochte surged to victory in a race featuring all the four Olympic strokes while Phelps faded to fourth.
Lochte was awarded the anchor leg in the 4x100 metres freestyle relay on the following night, even though he is not a specialist sprinter. The United States led into the final leg where Yannick Agnel swam a full second faster than Lochte to snatch a famous victory for France.
This time it was world champions Australia, with the 100 metres freestyle favourite James Magnussen swimming the first leg, who finished out of the medals.
Phelps did, though, swim the fastest leg for the Americans and is now just one medal short of Soviet gymnast Larisa Latynina's record haul of 18.
After both races, Phelps' team mates cautioned against writing off prematurely the greatest swimmer in the history of the Games. His best events are still to come but the question remains whether, with retirement beckoning at the end of the Olympics, Phelps can still summon the desire to defeat younger and hungrier opponents.
For the first time at a Games since the Soviet Union boycotted the 1984 Los Angeles Olympics, the all-around women's gymnastics world champion will not feature in the final.
Seventeen-year-old Jordyn Wieber dissolved in tears after one step outside the marked area in the floor exercise allowed her room mate Aly Raisman to take the second American place in the final.
South Korea's men, attempting a fourth consecutive Olympic team archery gold medal, set the first two world records of the Games in the ranking round on Friday before the opening ceremony.
But they lost to the United States in the semi-finals, who in turn were defeated by Italy in the final at Lord's Cricket Ground in central London.
Another country with a proud history in its chosen sport has faltered. Hungary have ruled water polo, winning a literally bloody battle against Russia at the 1956 Games.
They now face an immense struggle to win a fourth successive title after losing to Serbia in a group match.
New heroes are emerging to replace the old. A nerveless Michele Frangilli, needing a perfect 10 with his shot to win the archery gold for Italy, hit the centre of the target.
Chinese schoolgirl Ye Shewin, 16, swam the final freestyle leg of the women's 400 metres individual medley faster than Lochte after she was a body length behind after the breaststroke.
Host country fans are finding out how tough the Olympics are. Expectations were high on Saturday when the British cycling team set out to guide Mark Cavendish to the gold medal in the men's road race.
Instead, the other contending nations understandably refused to help make the pace and Cavendish, the world champion, finished 29th.
On the other hand, home support boosted Rebecca Adlington to the bronze medal on Sunday evening after the defending champion scraped into the women's 400 metres freestyle final and was condemned to lane eight.
"That's what being at home is all about," Adlington said. "It's what helped me secure that bronze medal. I heard the crowd, thought this is unbelievable, put my head down and went."
