The Springboks were edged by the Irish in Paris and still need to secure a quarter-final berth. Five points against the Tongans should do that, but anything less will leave them sweating over a knockout place.
Ireland head Pool B with 14 points, followed by South Africa on 10 and Scotland with five, though the matter must still play minnows Romania and, crucially, the Irish in Paris on October 7.
"We know we need to get the five points against Tonga, so we will go into the game with the mindset that we need to get a bonus point," Smith told reporters at the team's base in Toulon on Tuesday.
"Tonga are a physical team, so to play against them will be great to know our physicality is also at the highest level.
"We need to keep that standard of intensity in every game we play. We will need it this weekend. They are a physical side and are hard to play against. From minute one we need to play hard and take our opportunities."
Missed opportunities is the theme from the Bok camp following their Ireland defeat, not just the 11 points they did not take from the kicking tee, but also inaccuracy with ball in hand when in the Irish 22.
Smith knows they cannot afford to make similar errors should they advance to the knockout rounds.
"It was a disappointing loss for us, we have reviewed it and we had a lot of opportunities which we did not use. We will have to make sure that doesn't happen again because from now on, we can't have any more slip-ups," Smith said.
"The breakdown is certainly one of those phases where they slowed things down and the ball (came out) one or two seconds slower than it should have. That takes away the chance to score a try.
"But we are excited to know that we created those opportunities and should we play them again, we have to be sharper and use them. That is what we will need to do to win the World Cup."