Host Nation Great Britain finished third after Australia withstood a late push. They expect to progress to the final from Tuesday's repechage.
Meanwhile, Great Britain's Zac Purchase and Mark Hunter delivered a powerful statement of intent as they roared back into form with victory over New Zealand in their Lightweight Double Sculls heat.
The reigning world and Olympic champions were at rock bottom just five weeks ago after finishing sixth for the second World Cup regatta in a row.
But Purchase and Hunter have built a reputation for peaking at the biggest events, having overcome turbulent seasons to win the World Championships in both 2010 and 2011.
And Hunter believes they are on the road to doing the same at London 2012 after beating the form pair of Storm Uru and Peter Taylor en route to the semi-finals.
'This is the Olympic Games and it was important to come here and kick off with a good start, put the doubters at rest and show that we're back,' Hunter said.
'We hate losing more than any others. We go away and work hard and come back again and that shows class in my eyes. There is a sense of pride there.
'We've had a pretty disappointing season so to go out there and execute like we did in the heat showed what we can do - and there's a lot more to come.'
The world champions led their chief rivals from the start and held off a late charge to win by half a length, in 6:36.
New Zealand also suffered defeat to Great Britain in the women's Lightweight Double Sculls heats.
Sophie Hosking and Katherine Copeland, World Championship bronze medallists last year, made a lightning start and pulled clear in the second 1000m in a supreme exhibition of sculling.