Despite spending a lot of their regattas in the winning streak experimenting, they couldn't be beaten.
The loss came in a shortened two-day regatta and at the end of a sustained Rio training block.
Tuke said they were not fussed about their winning stretch.
"It's never been about the run for us," he said.
"There have been certain regattas that we've wanted to perform at and we've done okay at those.
"If we hadn't, we'd be having a good long look at it."
Transtasman rivals, and defending Olympic champions, Nathan Outteridge and Iain Jenson took out the South American championship and loom as the biggest threat to the Kiwi pair.
Germany, Great Britain and Denmark also will feature as challengers for the gold.
The four-time world champion Kiwis, crowned world male sailors of the year in 2015, don't spend any time analysing opponents, rather they chose to focus on conditions.
Guanabara Bay is known for its volatile conditions, which could turn the regatta into a virtual lottery.
Tuke said they've spent more than 100 days in Rio in the past three years, contesting five regattas, but haven't made much progress in grasping the shifty winds and wild currents.
"We've got a playbook for racing here but we don't rely on it," he said.
"The thing you've got to know is that it will change, sometimes a lot."
The potential for getting caught up in rubbish and other debris in the polluted water could be an issue for the pair, but Tuke said they had worked out how they would counter the problem, should it arise.
"If it is slowing us down, we'll remove it. If not, we'll plough on," he said.
Tuke confirmed the regatta will be the last in a 49er for at least a year as they eye another form of sailing when they join Team New Zealand's challenge for the America's Cup next June.
Their races begin on August 13 and ends with a medal race on August 19.
