Fernando Santos admits it would be foolish for his Greece team to under-estimate Costa Rica, who will look to make history in Recife.
When Costa Rica were placed in Group D last December, there seemed little hope of the Central American nation making it through to the second round.
But six months later, they took everyone by surprise, outsmarting three former world champions to progress to the last 16.
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Jorge Luis Pinto's men stunned Uruguay 3-1 and then beat Italy 1-0 to send England out of the tournament with one match to spare.
Their progression has made Greece wary of what awaits them in the Arena Pernambuco tonight.
"We have to pay a lot of attention in this game, they came out top in the 'group of death'," Greece coach Santos said. "As a result we cannot allow ourselves to take them lightly."
With so much riding on the match, Costa Rica could be forgiven for adopting a more cautious approach in Recife.
And if there is no winner after 120 minutes, the Central Americans are prepared for what comes next.
"Penalties could play a crucial role at any moment," assistant manager Luis Marin said.
"So we have been practising them for the last few days and we know who will take them."
Costa Rica are expected to name the same XI that beat Italy in their penultimate group game.
Greece, who qualified for the last 16 thanks to a last-minute penalty by former Celtic striker Georgios Samaras against Ivory Coast, will be without midfielder Panagiotis Kone through injury.
But goalkeeper Orestis Karnezis should be fit despite suffering a knock against the Africans.
Santos must decide whether to recall captain Kostas Katsouranis, who was suspended for the Ivory Coast match.