Few gave them a prayer. Drawn in a group featuring three World Cup winners with seven titles between them, pretty much everyone expected Costa Rica to be heading home early from Brazil 2014. Everyone, that is, except the team itself.
Goalkeeper Keylor Navas' claim before the tournament that Italy, England and Uruguay were "three great teams fighting for one place" in Group D sounded far-fetched. It turned out to be spot on.
Costa Rica stunned Uruguay by coming from a goal down to beat the South Americans 3-1 in their opening match. Then they beat Italy 1-0 to seal a second-round spot before a goalless game against England saw them top the group and secure a last-16 meeting with Greece.
Win it and they will have made history, having never gone beyond the last 16 of a World Cup - an achievement los Ticos managed at Italia 90. So how have they done it?
Often overshadowed by traditional Concacaf powers Mexico and United States, Costa Rica finished second in World Cup qualifying behind USA to seal a spot at Brazil 2014, winning all five of their home matches, drawing three on the road and losing just twice.
Goalkeeper Navas was La Liga's finest shot-stopper last season and has popped up at crucial times in this tournament, making vital saves in the games against all three rivals in the group stages.
Ahead of him, the side's five-man back line allows Jorge Luis Pinto's men to defend in numbers and attack on the break, with Oscar Duarte, Giancarlo Gonzalez and Michael Umana all impressing in the centre of defence.
Cristian Gamboa and Junior Diaz both like to get forward from the wide positions as attacks begin from the flanks and the forwards link up wide, with Celso Borges and Yeltsin Tejeda working incredibly hard in midfield to track back and also initiate counterattacks from deep. The front three of Joel Campell, Bryan Ruiz and Christian Bolanos adds pace, power and finishing ability in a team in which everything is clicking.
Ruiz scored the winner against Italy, but said: "It's not about Bryan Ruiz. It's about the team, the collective. We all work hard for each other and it's a group effort."
Those thoughts were echoed by coach Pinto ahead of the game against England. "There are no secrets," he said. "We have worked on mental and emotional aspects. Our strength is in our togetherness and our unity."
Pinto, however, is a coach who leaves absolutely no stone unturned as he seeks improvement for his side. "I learned from watching Italy in five World Cups," he said last week. "I'm taking advantage of that now, for example with the zonal marking we use."
When they need to, Costa Rica defend with seven men in a colossal collective effort as the two midfielders drop deep to aid the five-man back line. At times, such as against Italy, they have switched to man-to-man marking as well, while their attack is all about exploiting spaces on the break and the midfield also looks to switch play when needed to shake off their rival markers.
It's extremely effective and with three world champions already out at their expense, Greece - and future rivals, if they advance - will do well not to underestimate this team that has surprised the world already.