Goalkeeper Navas kept out former Levante team-mate Theofanis Gekas' spot-kick in a penalty shoot-out against Greece on Sunday after extra time ended with the scores locked at 1-1.
The stop allowed Michael Umana to step up and send 10-man Costa Rica through to the last eight for the first time and set up a clash with the Netherlands in Salvador on Saturday.
And Navas insists that Jorge Luis Pinto's men, who have already beaten Italy and Uruguay in the tournament, will be determined to spring another surprise.
"This is a huge achievement for us," he told FIFA.com. "We are thrilled to bits and we're doing something massively important for our country. We need to keep this feeling going.
"The Netherlands are a great side and one of the biggest teams in the world, but we're going be just as determined when we play them and show the same character. We'll be going out to win."
The 27-year-old knew Gekas' penalty routine from their time together at Levante and used that knowledge to make the all-important contribution.
"We played together at Levante and I remembered what he used to do in training," he added.
"I was confident he wasn't going to change the way he takes his penalties, and in the end I was able to keep it out.
"It's a fantastic feeling. Thank God I was able to stop it. My team-mates also deserve praise for putting all their penalties away. I took confidence from that, and it definitely helped me.
"[Before the penalties] we got together in the centre of the pitch and had a chat. The message was that we'd worked so hard to get that far, and we were convinced we could win.
"I think everyone felt that deep down in their hearts and that's why we got there in the end."
