Forty-year-old British rower Katherine Grainger described a fourth silver medal in five Olympics as her "greatest achievement" Thursday, as dominant New Zealand pair Eric Murray and Hamish Bond stretched an eight-year unbeaten streak.

After two of the first five days of the disrupted Rio regatta fell victim to blustery conditions, the sun shone on a hectic first medals day when six golds were handed out at the picturesque Rodrigo de Freitas Lagoon by Copacabana beach.

Germany also struck double gold in the men's and women's quadruple sculls, Croatian brothers Martin and Valent Sinkovic claimed the double sculls and Switzerland the lightweight men's four.

Grainger was agonisingly close to defending her double sculls title with Victoria Thornley, but they were pulled in by Polish pair Natalia Madaj and Magdalena Fularczyk-Kozlowska in the final 500 metres.

However, a fifth Olympic medal made Grainger, who has a degree in homicide, Britain's most decorated female athlete.

And the Scot hailed a fourth silver after also finishing second in Sydney, Athens and Beijing having taken a two-year sabbatical after finally landing gold in London.

"I don't think anything would eclipse London," Grainger told the BBC.

"There were many dark, dark, days after my comeback but I would say this is my greatest achievement."

Grainger and Thornley were only added to the GB rowing team in late June after failing to be selected for the women's eight.

"At the beginning of the regatta you'd have taken that with both hands. I'm delighted."

Kiwi superstar duo Murray and Bond had no such trouble, retaining their Olympic title in the men's pair to stretch a remarkable unbeaten record since they teamed up in 2009 to 69 races.

The six-time world champions were well off the world record time they set in London, but still finished nearly three seconds ahead of South African pair Lawrence Brittain and Shaun Keeling with Italians Giovanni Abagnale and Marco Di Costanzo taking bronze.

"If we put out a good race and we don't win we'll take it, but we don't want to let the New Zealand rowing team down," said Bond after landing New Zealand's first gold of the Games.

"We still have to execute, but the hard work is done in the four years leading up to this.

"It's the nature of being undefeated, because we can't exceed expectations - only match them."

There was plenty of brotherly love for the Sinkovics as the siblings, born just 15 months apart, stormed past halfway leaders and silver medallists Lithuania with Norway picking up their first medal in bronze.

"My wife is here but my parents couldn't afford to come," said an emotional Martin Sinkovic. "These are beautiful tears!"

Valent dedicated the triumph to the pair's parents.

"I can't wait to hear our parents voices because they have sacrificed everything for us to achieve this," he said.

Germany surged up the medals table as after winning just one gold on the opening five days of full competition, they bagged the first two of the regatta in a matter of minutes.

The men's crew of Phillipp Wende, Lauritz Schoof, Karl Schulze and Hans Gruhne defended their title in the men's quadruple sculls

Moments later it was the women's turn as Annekatrin Thiele, Carina Baer, Julia Lier and Lisa Schmidla claimed gold.

Switzerland won the final gold of the morning as Lucas Tramer, Simon Schuerch, Simon Niepmann and Mario Gyr held off Denmark and France in the men's lightweight four.

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