The Euro 2004 champions pulled off the shock of the tournament by beating Russia 1-0 in their final group game to qualify from Group A alongside the Czech Republic.
And Fernando Santos’ side have been rewarded with a knockout tie against Joachim Low’s men, who are the only team in Euro 2012 to have won every match thus far.
“They are a very strong team, that’s true. They are very big favourites,” Sifakis told Uefa’s official website.
“We Greeks have an attribute the others don’t have, who have persistence and talent.
“Unfortunately, we do not have the infrastructure they have in Germany for instance. But I think our persistence helps us overcome all obstacles.
“They are the favourites, but Russia were also the favourites.
“We know our potential, we know what we have to do [and] we know what we must not do. So we are not stressed by the fact that we are playing against a strong team that are the favourites.
“Stress is on their side, not ours.”
Injuries meant Sifakis spent much of last season on the sidelines, but he is delighted to have recovered in time for the tournament in Poland & Ukraine.
“The day I went into hospital to undergo surgery, this was the first goal I had on my mind for when I would recover,” the 27-year-old added.
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“I finally managed to recover earlier than predicted and started training with the team towards mid-to-end January.
“I thought I would be here, I really wanted it and I’m delighted to have finally made it. That was a very difficult six-to-seven month period for me, being absent for all that time.”
Sifakis started the tournament as second choice, but came on during the first half of a 2-1 defeat to Czech Republic following an injury to Kostas Chalkias.
Since the arrival of the Aris Thessaloniki shot-stopper, the Greeks have yet to concede a goal in the tournament, but he insists his sudden introduction has not been easy.
“Look, the truth is that in the match against Czech Republic, it was very difficult for me to go on in the 23rd minute,” he confessed.
“When we were already losing 2-0 and our chances for the match were significantly fewer.
“Our feelings were very weird, especially for me since I came on from the bench in such an important match.
“In any case, I’m very pleased that the team qualified for the quarter-final.
“We never take each player’s performances separately into consideration. Each one of them plays their own distinct role on the pitch, and we all try to give our best so that we can win in the end."
Sifakis went on to express his belief that the nation can repeat the surprise success of eight years ago, but only if they work as a team.
“I think it’s the secret for the big successes of this team," he continued.
“If we go back to 2004, we can’t possibly remember a single player standing out from the other 22.
“All players who participated fought like a single man and we managed to win the Euro.”