With 500 metres to go Richard and Pete Chambers, Chris Bartley and Robert Williams were lying in fourth for Team GB, but they finished strongly and looked like they might have snatched gold with 100 metres left.
But the South Africans came powering through to snatch gold from their grasp by a tiny margin.
“Fair play to the South Africans, they won it fair and square,” said Pete Chambers. “We’re gutted – we were unlucky to get silver with those conditions. We’re delighted with silver but we wanted gold.”
They struggled to keep up with the early pace set by defending champions Denmark, but as the race headed towards the business end the four settled into a quick rhythm themselves.
“We were just fighting, fighting through the whole thing to get ourselves back in contention,” added Richard Chambers.
But Williams remained upbeat despite the disappointment: “You can’t be upset when you win a silver medal with 30,000 people cheering you on. It was a fabulous experience.”
And the scintillating pace took its toll on strokeman Chris Bartley, who had to be helped onto the podium after the tremendous effort.
Earlier, Bill Lucas and Sam Townsend fought valiantly in the final of the men’s double sculls, pushing for a medal for much of the race.
But the fast pace and power of the New Zealand pair, who romped home to add an Olympic gold to their world championship win in 2011, with Lucas and Townsend ending in fifth.
Elsewhere, the women’s eight of Olivia Whitman, Annabel Vernon, Victoria Thornley, Louisa Reeve, Natasha Page, Lindsey Maguire, Kate Greves and Jessica Eddie were in action in the last final of the day.
But they were unable to match the stunning performance of their male counterparts just a day ago as they finished fifth, with the USA retaining their title in comprehensive style ahead of Canada and the Netherlands.
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BIOGRAPHY: Daniel Sturridge
BIOGRAPHY: Kepa Arrizabalaga