Tough-talking swimmer James Magnussen makes his Olympic debut Sunday as the brash Australian spearheads his country's bid to dethrone the United States in the men's 4x100m freestyle relay.
In Magnussen, nicknamed "The Missile", and James Roberts, Australia have the two fastest men in the world this year by a wide margin. They are joined by former world record-holder Eamon Sullivan and world champion Matthew Targett.
Nathan Adrian, winner of the individual 100m free at the US trials, lags a second outside Magnussen's top-ranked time of 47.10sec – the fastest since high-tech bodysuits were banned.
But the Americans, who have won eight of the 10 Olympic gold medals awarded in the event, say they aren't going down without a fight with Michael Phelps desperate to make up for a disappointing first day at the Games.
Phelps's 400m individual medley showdown with Ryan Lochte on Saturday turned out to be a no-contest.
The 16-time medallist, who won a record eight titles in Beijing and needs just three more medals to overhaul Soviet gymnast Larisa Latynina's all-time best of 18, slumped to fourth behind his compatriot.
Teenage American swimmer Missy Franklin, competing in seven events in London, begins her individual programme in the 100m backstroke.
Meanwhile, the United States' collection of NBA multi-millionaires, who are expected to cruise to a successful defence of the basketball title, start their campaign against France.
With LeBron James, Kobe Bryant and Carmelo Anthony leading the way, the US are aiming for a 14th gold in 18 Olympics.
Bryant is so confident of victory that he insists the 2012 line-up is even better than Michael Jordan's fabled Dream Team of the 1992 Barcelona Olympics.
"People who think we can't beat that team for one game, they are crazy. We can beat them one time," said Bryant.
Wu Minxia launches China's sweep for all eight diving gold medals with her bid for a third straight synchronised three-metre springboard crown.
The 26-year-old can leave the Games as the most successful Olympian in the sport along with the retired Guo Jingjing if she wins both her springboard events and so adds to her current tally of four medals.
China won seven of the eight titles in Beijing four years ago.
Over at Wimbledon, Maria Sharapova takes on Israel's Sharar Peer as the women's tennis first round is completed.
Former world No 1 Novak Djokovic and home hope Andy Murray, who was knocked out of the doubles on Saturday, are also in action.
There are 14 medals up for grabs on Sunday.
In women's cycling, Dutch rider Marianne Vos of the Netherlands starts as favourite to take the title won by Britain's Nicole Cooke in Beijing four years ago, but the host nation are desperate for a first medal.
Two golds are also up for grabs in weightlifting, judo and shooting with one each in fencing and archery.
Away from the action, organisers will face questions over why there were significant numbers of empty seats on the opening day at several venues, including tennis and swimming, despite overwhelming pre-Olympics demand for tickets.