South Africa's rowing team said they were motivated by their countrymen who had shone in the swimming pool after they secured the country's third gold medal of the London Olympic Games.

The quartet of Sizwe Ndlovu, Matthew Brittain, John Smith and James Thompson won a nail-biting men's lightweight four final on Thursday afternoon.

"The other night we watched Chad (Le Clos) racing and he was a little bit behind and he just came for it at the finish line," Brittain said.

"We said to ourselves that's how we are going to do this thing - put ourselves in the mix in the first half and in the second half we are going to moer (hit) them at the end, so that was our plan. It was just amazing."

The South Africans started conservatively, going through the 500 metres mark in third place, and dropped back to fourth by the 1 500m mark.

In the final 500m it seemed as if the team could challenge Great Britain for the bronze medal.

With every stroke, however, they inched closer to the host nation, Australia and Denmark.

Over the final 250m they surged past the other three boats to win by a quarter of a second in a time of six minutes, 02.84 seconds (6:02.84).

Britain secured silver, 0.25 seconds behind the SA quartet, with Denmark bagging bronze a further 0.07 seconds behind.

Their medal adds to golds secured earlier in the week by swimmers Cameron van der Burgh and Le Clos.

"In the race we were just thinking of going harder and we just started shouting 'gold'," Brittain said.

"We said the whole season we only had one goal - when anyone of us says that, then everyone has to say 'gold'."

While their victory may have come as a surprise to most, Thompson said they had believed they could finish on the podium.

"We had firm belief that we could certainly pull off a medal," Thompson said.

"We've only lost to the Chinese before this, so for us we were in a good place the whole year.

"So it wasn't such a surprise to us."

The crew received a confidence boost before the Games when they bagged the silver medal at the World Cup Regatta in Switzerland, where they finished second behind the Chinese.

Thompson said they had worked hard to be in top form at the global showiece.

"It is really special to go into the Olympics in such good form and such a good year," he said.

"The whole year we've just been getting better and better.

"We didn't miss one piece of training and we didn't have one injury.

"It was a build-up to dream of and that preparation showed here."

Ndlovu, who made history as the first black person to win an Olympic medal in rowing, said they had backed themselves over the last quarter of the race.

Ndlovu became only the second black South African to earn an Olympic gold medal, behind 1996 marathon champion Josiah Thugwane.

"We knew we were stronger than these guys over the final 500m or 200m or so," Ndlovu said.

"We have our gears there, we went, they couldn't hold us, and we went through."