The African side earned plaudits for a brave performance in their opening 58-14 loss to world champions New Zealand.
"If we start slow against them, Tonga will punish us," captain Jacques Burger told reporters on Monday.
"It remains our goal to win -- it's not impossible. We know how tough Tonga are as a rugby side. We want to win. Not to compete, but to win.
"It'll be tough but we want to do everything in our power to do that."
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Tonga lost 17-10 to Georgia following a lacklustre display in their first game but Namibia will not be taking them lightly.
"Tonga are a really dangerous side," Burger said.
"They've got a very physical way of playing. When we do kick we have to make a good chase. We have to chase really hard. What we lack in skills, or speed and power, we have to make up in desire."
Coach Phil Davies has made eight changes to his team who are bidding to secure Namibia's first World Cup win at the 17th attempt.
"We try to get the best 15 on the pitch and want a high-intensity performance with lots of belief, effort and courage," he said.
(Reporting by Ed Osmond; editing by Justin Palmer)
