Top-ranked Ireland claimed a 13-8 statement victory over the Springboks in a high-octane heavyweight World Cup clash that lived up to the hype at a sweltering Stade de France.
"We're in a good place but we can't get carried away, we need to keep our feet on the ground," Sexton told reporters, saying his team's focus needed to remain on winning their final Pool B game against Scotland on October 7.
"When you're world number they use that as motivation," he added.
The game was arguably the most hard-fought of the tournament so far with the finest of margins separating the two sides.
Ireland coach Andy Farrell said his team would benefit from winning such a high-intensity game and keeping their emotions in place as they move forward in the World Cup.
"How we managed to stay on point mentally was fantastic, how we kept our heads, getting those couple of penalties at the end when it really mattered, is really the big plus side of our performance," he said.
"It’s wonderful to win, but there was not much in it between the two sides,
"We will know what that feels like further down the line, to learn those lessons with a win. When you love defending as much as we did in that game, that stands us in good stead.
"We will need to be better as the competition goes on. We’ve got very good at not getting too emotional and staying on task."
But while praising the team's mentality, Farrell and Sexton also hailed the impact of the thousands of travelling fans who cheered and roared them on and in jubilation and sang "Zombie" by Irish band the Cranberries in unison at the final whistle.
"I've never seen a crowd like that," Sexton said, referring to the 30,000 or so Ireland supporters in the stadium. "They were insane and gave us the lift we needed. We play for them and they gave us the edge tonight I think."
