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China's Jingli Duan claimed bronze while New Zealand's Emma Twigg, who had been expected to be Brennan's strongest challenger, came in fourth.
Brennan has dominated the class since taking bronze at London 2012. But she almost missed out on the final in Rio, finishing 14th in the heat rankings, only just making the automatic qualifying.
And she was delighted to finally win gold, not least to bring an end to her Olympian husband, Scott’s bragging rights, after he took double sculls gold at Beijing 2008.
“I've imagined this so many times,” she said. “So much work has gone into this over the past 11 years and I've had so much support from my coach and team-mates. My husband has a gold medal so now we're even!”
In the men’s single sculls, defending champion Mahe Drysdale of New Zealand beat Croatia's Damir Martin in a photo-finish to win gold. The race had been billed as a showdown between the Kiwi and his arch-rival Ondrej Synek of the Czech Republic, who took the bronze medal.
Martin took an early lead, with Drysdale and Synek in hot pursuit, but the New Zealander moved ahead in the last section. At the line, however, the Croatian shot forward with a final lunge, only for Drysdale to be given the nod by a margin of 5,000ths of a second.
“That was absolutely amazing,” said Drysdale after his photo-finish victory. “I got the feeling (Martin) just passed me and I just chucked in a few short ones in desperation. It was not the way you want to finish, but to come away with that result was fantastic.”
Meanwhile, Martin was left feeling that there was nothing more he could have done to overhaul his rival. “It's one of the perfect ones, where I did 100 per cent, even more. The last 100 metres was incredible, the shouts from the spectators were crazy.”
In the women’s eight, the US crew powered to the gold, to win their third back-to-back Olympic title and extend a 10-year unbeaten run. The Americans finished in 6 minutes 1.49 seconds, 2.49 seconds ahead of Britain.
The American eight - Emily Regan, Kerry Simmonds, Amanda Polk, Lauren Schmetterling, Tessa Gobbo, Meghan Musnicki, Ellie Logan, Amanda Elmore and Katelin Snyder - took the lead in the third section after Canada and the Netherlands made the early running. The British boat, which in the middle 1,000m had been back in last place, came good in the final stretch to claim silver, holding off a late surge from Romania, who took bronze.
It was a second gold for Musnicki and Logan, the only two members of the crew left over from the winning boat at London 2012.
Their team-mate, Polk was in triumphant mood, as she waxed lyrical about the US crew’s latest gold. “I was hyperventilating. It was so amazing,” she enthused. “To do this with everybody in our boat is so special. Fifteen years of rowing, 30 years of family support, and an awesome group of girls who push me every day. No margin is big enough, no stroke is hard enough, but the important thing is we did this together.”
Meanwhile, Great Britain’s silver represented a first ever medal in the event, and was particularly poignant for 35-year old crew veteran Sarah Houghton who had been part of the eight for over a decade.
“We did this for all the girls who have been in this boat over the last 20 years. They helped make this possible; they know who they are,” she said
Great Britain’s men went one better than their female counterparts, the final event of the rowing regatta, storming to gold over reigning Olympic champions Germany.
The British boat, made up of Scott Durant, Tom Ransley, Andrew Triggs-Hodge, Matthew Gotrel, Pete Reed, Paul Bennett, Matt Langridge, William Satch and Phelan Hill finished the 2000m course in 5 minutes 29.63 seconds. The Germans claimed silver in 5:30.96, while the Netherlands eight finished third on 5:31.59.
The British eight led from start to finish - by as much as 2.5 seconds at one point, which according to William Satch was precisely what they had planned. “That was our plan, to not look back and keep going and going,” he said. “I couldn't believe we had actually done it when we crossed the line."
Germany’s Eric Johannesen, was gracious in defeat: “It was a very tough race. Our expectations were the gold medal. The British are the stronger team, they rowed a very good race, and now we have silver. I think we need a little time to think about it, to see if we are happy with it.”
The British topped the final rowing medals table with three golds, while Germany and New Zealand finished with two apiece.