They beat Japan 5-3 on penalties after a goalless extra time draw at Loftus Versfeld on Tuesday, becoming the fourth South American team to reach the quarter-finals.
However, unlike the high-scoring Argentina and Brazil, coach Gerardo Martino's team netted a mere three goals to win their group, with none by a forward.
"I wouldn't like to be unfair to the forwards because, to score goals, the ball should reach them more cleanly and in advantageous positions," the Argentine said.
"We had some chances but they were tight. I think our future rivals will let us play differently, not having to take the game to them and that's more convenient," he told the post-match news conference.
"In three of the four World Cup matches we had to take the game to our opponents and that's difficult for a team like ours. (Spain) will give us more space."
Spain, who meet Paraguay at Ellis Park in Johannesburg on Saturday, will certainly play more openly than Japan, who sat back and tried hitting the South Americans on the break.
Vicente del Bosque's side showed their credentials as one of the favourites by getting past a tough Portugal side on Tuesday and Paraguay might need to hold out for another impeccable set of penalties for an upset.
PARAGUAYAN PRIDE
Even if they lose, Martino and his team will still be able to leave South Africa proud to have given Paraguay their best World Cup.
The Argentine had said long before the finals that Paraguay had the players to break their traditional mould of a side with a solid back line and potency in the air.
They are more versatile up front even after the loss of shooting victim Salvador Cabanas, a key player in the qualifiers, but they lack the quality of a David Villa, Xavi or Andres Iniesta.
"I think we can leave this World Cup with our heads very high," said striker Nelson Valdez.
"People say that five qualify out of 10 (South American Football Confederation teams) and it's easy but as you can see nothing's easy in South America and now we're in the best eight and that has to be celebrated," he told reporters.
"I don't think we're 100 percent aware yet of what we have achieved."
