Defending champions Iain Percy and Andrew Simpson tightened their grip on the Olympic Star class title on a dominant day for British sailors in Weymouth.
The pair came first and second in their two races in big waves and strong winds to extend their lead to nine points over Brazil and Sweden.
Ben Ainslie put his quest for a fourth Olympic gold medal firmly back on track with a first and a third to close to within three points of long-time leader Jonas Hogh-Christensen of Denmark.
GB 49er duo Stevie Morrison and Ben Rhodes raced to two firsts to climb from fifth to second, 11 points adrift of the Australians.
And the men's 470 regatta kicked off with Britain's Luke Patience and Stuart Bithell grabbing a second and a first to take the early lead. Windsurfer Nick Dempsey also had a good finish on a highly successful day for Britain with victory in race two to stay fourth overall, two points off silver.
"It couldn't have gone much worse in the first race. I made a mess of the first upwind and then battled my way up through the fleet. In race two I made a good start for once and took the opportunity so had a good sail," said Dempsey.
Percy, who is chasing a third gold medal in four Games, teamed up with childhood friend Simpson ahead of Beijing and the pair fought back from a slow start to clinch gold from Brazil and Sweden in China.
In Weymouth, the Britons have finished in the top three in seven of their eight races, barring an 11th in the opener.
Percy and Simpson's main rivals, Beijing runners-up Robert Scheidt and Bruno Prada, had a third and a fifth, while Sweden's Fredrik Loof, who won bronze four years ago, and new crew-mate Max Salminen scored a fourth and a first to draw level on 22 points with the Brazilians.
"It's a big game, the same medallists as last time all fighting at the front," said Simpson, who is chasing his second straight Olympic gold.
"It's pretty tough racing and pretty intense. If you make a mistake you're going to get punished. We've got a tiny cushion but you've got to keep chipping away."
Ainslie was angered by Hogh-Christensen and Dutch sailor Pieter-Jan Postma suggesting he had hit a mark in race two in big waves and strong winds on Weymouth Bay.
The 35-year-old Ainslie, who says his rivals "teamed up" on him, denies he hit a mark but took a penalty turn as a precaution. But he warned they had fired him up for the remaining two races and the double points-scoring medal race on Sunday.
Morrison and Rhodes went to Beijing as world champions and among the favourites but struggled in the light airs to finish a disappointing ninth.
In Weymouth, they started slowly with two 12th places, while a capsize on day two did not help their cause. But they have since hit back with a fourth, second, first, first to narrow the gap on Australia's Nathan Outteridge and Iain Jensen, who were 10th and sixth on Thursday. The 49er fleet has seven more races before next Wednesday's medal race.
"It doesn't happen like that very often in our class, you've got to enjoy these days," said Morrison.
"We said at the start of the week not to panic. It helped that the Aussies didn't have their best day. But they have had their wobble and will bounce back. It's all still to play for."
Olympic debutants Patience and Bithell were expected to be in the shadow of Australia's world champions Malcolm Page, who won 470 gold in China, and helm Mathew Belcher.
But the Britons, who were runners-up at the Worlds in Perth last year, ignored the form book to excel in the afternoon sunshine in front of the huge crowd on the ticketed "Nothe" spectator area.
"That was champagne yachting," said 25-year-old Scot Patience. "We're living the dream. It's great fun. What's not to love? It is a nice way to start, but the mistakes will come."
In the women's windsurfing, Britain's Beijing bronze medallist Bryony Shaw slipped one place to seventh with a sixth and an eight.
Lucy Macgregor's match-racing crew beat Sweden in their only race to remain in a tie for fifth after nine of their 11 round-robin matches before the top eight go to the knockout stage.