Great Britain's Katherine Grainger and Anna Watkins broke the Olympic record by nearly five seconds to book their place in the double sculls final.
Watkins and three-time Olympic silver medallist Grainger, unbeaten since they teamed up in 2010, won their heat in six minutes 44.33 seconds.
British men's four
won their heat to reach Thursday's semi-finals.
"Kath Grainger and Anna Watkins seem so relaxed. They are ready for this. That was the best I have ever seen them scull. They are favourites for this and they are looking a class above everybody else."
GB men's eight
held off a fast-finishing Canada to win the repechage and qualify for Wednesday's final.
Grainger, 36, and Watkins, 29, produced a superb display to win their 21st race in a row and will be favourites to win gold in Friday's final at 10:30 BST.
The reigning world champions were quickly away and had opened up significant clear water by the 1,000m halfway point.
The British pair's time smashed the previous Olympic best set at the 1992 Barcelona Games by Germany. The result was even more impressive given that they appeared to ease off in the final 500m before cruising across the line to the delight of the home crowd.
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Andy Triggs Hodge
“Today, we opened the door. In the semi-finals, we've got to look through it. And in the final, we've got to walk through it. ”
Grainger said: "I'm really pleased with our first event. You can hear the crowd but you feel it in your body, pulsing through you.
"We're very lucky to have this incredible support from around the country. It lifts you like nothing else.
"The plan for the next four days is feet up... not! We've got an adrenalin high now and we're both aware we can enjoy it for the next few hours, but we need to bring it back down. We'll head back to training and a dull lifestyle to get everything in place for Friday."
Their main rivals for gold, Brooke Pratley and Kim Crow of Australia, won their heat in a time four seconds slower than the Britons.
The British men's four - Pete Reed, Andy Triggs Hodge, Tom James and Alex Gregory - arrived at Eton Dorney on the back of defeats by Australia in the semi-finals and final of the last World Cup in Munich.
But they have appeared relaxed in the build-up to this event after a tough six-week training camp and sent out a message of intent with a strong opening heat.
The British looked incredibly comfortable all the way through the race, clocking five minutes 50.27 seconds to win by just over a length.
Australia had earlier demonstrated their strength by setting an Olympic best of five minutes 47.06 seconds - taking 1.4 seconds off the previous record set by Germany in Athens eight years ago - to progress.
"We all know it's going to be nip-and-tuck between the Brits and the Aussies. I think Australia grabbing the Olympic record will ruffle our guys and it should help them. I'm very happy with our team. Everything seems to be coming together at the right time and everyone seems very classy and relaxed."
"It was a great start for us," Britain's Gregory told BBC Sport. "We had a good margin on the field so we didn't need to show everything we have got.
"It doesn't matter about the times, we're just focused on what we're doing."
The British men's eight were disappointed with a bronze in the final World Cup meeting before the Games.
But the return of Constantine Louloudis, who missed the World Cup season with a back injury, inspired the crew - also made up of Alex Partridge, James Foad, Tom Ransley, Ric Egington, Moe Sbihi, Greg Searle, Matt Langridge and cox Phelan Hill - to a strong second behind world champions Germany in the heats.
They made another step forward by dominating the repechage from the off, beating Olympic champions Canada and achieving a time just one second slower than the Germany's best at this regatta.
They will race for gold on Wednesday at 10:30 BST.
Britain's women's quad
- Melanie Wilson, Debbie Flood, Frances Houghton and Beth Rodford - produced a brilliant fightback in the repechage to qualify for the final (Wednesday, 10:20 BST) in third place.