* U.S. see off Canada to win women's eight final
* South Africa grab first Olympic rowing gold
* New Zealand triumph in men's double sculls
DORNEY, England, Aug 2 (Reuters) - The United States powered to gold in the flagship women's Olympic eight on Thursday after fighting off fierce rivals Canada to maintain their stunning six-year winning streak and cap a thrilling day of racing on Dorney Lake.
The dominance of the U.S. women contrasted with the other two Olympic finals of the day, when New Zealand sprinted through the field to grab gold on the line in the men's double sculls while South Africa snatched victory in a thrilling men's lightweight four final.
In the eight, the American crew took a half a length lead over Canada and the Netherlands in the early stages before holding on as their neighbours came back at them before 25,000 roaring fans packed into the course to the west of London.
"This was awesome, at the end I don't think I could have pulled one more stroke," Caroline Lind told reporters on the side of the lake. "We got the gold and that is what mattered."
New Zealand's Nathan Cohen and Joseph Sullivan, the double sculls world champions, had been sitting in fifth place for most of their race before they upped their rate and surged through a tiring field in an electrifying final 300 metres.
Italy's Alessio Sartori and Romano Battisti took silver and the experienced Slovenian crew of Luka Spik and Iztok Cop grabbed bronze after fading in the second half of the race. The Slovenians have now completed the set after winning silver in Athens and gold in Sydney.
But the best race of the day went to the South Africans who claimed victory by a couple of feet - a quarter of a second - with a breathtaking late burst to win a four-way sprint for the line in the men's lightweight four final.
It was South Africa's first Olympic rowing gold.
The victory, over Britain in silver and Denmark in third, prevented the host nation from scooping its third Olympic gold of the Games and denied brothers Peter and Richard Chambers from Northern Ireland the title.
The four of James Thompson, Matthew Brittain, John Smith and Sizwe Ndlovu, collapsed over their oars as they went across the line and looked to the big screens to see whether they had won.
The British boat later rowed past the packed stands to wave at fans and thank them for their support.
Ten minutes later a jubilant American women's eight crew stepped on to the podium to accept their medals and pose for cameras. They had earlier punched the air and splashed the water as they crossed the line.
The Americans had entered the Olympic regatta as hot favourites after winning the 2008 Games in Beijing and the last five consecutive world championships.
However, they were given an early warning of how difficult it would be to retain their title when they raced Canada in a world cup in Lucerne this year, winning by only 0.03 seconds.
Canada also posted the faster time in the heats earlier this week in their bid to win a first women's eight Olympic gold since they last won the title in Barcelona. They had to settle for silver and the Netherlands took bronze. (Editing by Matt Falloon)