After a surprising loss in its opening game, Spain regrouped to win its last two and capture the top spot in Group H.
La Furia Roja's reward is a meeting with rival Portugal - the only team yet to concede a goal in South Africa.
The last of the round of 16 matchups will be perhaps the most intriguing as the Iberian neighbors square off in the World Cup for the first time Tuesday in Cape Town.
Spain entered the tournament as one of the favorites after winning the 2008 European championship, but simply advancing past the group stage became a challenge after the team opened with a 1-0 loss to Switzerland.
David Villa responded with three goals, sparking La Furia Roja to victories over Honduras and Chile. Midfielder Andres Iniesta said the Spanish "lifted a weight off our shoulders" by holding off a 10-man Chilean squad for a 2-1 victory Friday to top the group.
"After the defeat to Switzerland we went through a bad time, but what is most important is that we've recovered and reached our objective," coach Vicente Del Bosque said.
Finishing first in Group H allowed Spain to avoid top-ranked Brazil in the round of 16, but Portugal certainly doesn't present an easy opportunity to advance even though the Spaniards are 15-5-12 in this all-time series. The Portuguese feel they deserve more respect after finishing fourth in the 2006 World Cup and going unbeaten in their last 19 matches overall.
"Everybody is talking about Spain all the time," Portuguese wing Simao said. "Sure, we have a lot of respect for them as well and they are the European champions. But that does not rule out what we've done over the last couple of years."
Portugal advanced to this point after a 7-0 win over North Korea and scoreless draws with Ivory Coast and Brazil. Needing only a tie against the Brazilians to finish second in Group G, Portugal clamped down defensively and secured its third straight clean sheet.
"We've only conceded three goals in our last 19 games and that's a record that means a lot to me and the team," goalkeeper Eduardo said. "We defend as a unit and we work hard for each other."
As Portugal proved against North Korea, its attack - centered around superstar striker Cristiano Ronaldo - can also be potent. Ronaldo just completed his first season with Real Madrid as a teammate of several Spanish nationals and an archrival of several others who play for Barcelona.
"I'm worried about Portugal, not (Ronaldo)," Del Bosque said. "We saw how they controlled Brazil. They didn't let them play their football and were strong on the counterattack."
Del Bosque may start 21-year-old Javi Martinez in the midfield if Xabi Alonso doesn't recover in time from a sprained right ankle. The status of Spanish striker Fernando Torres also is unclear as he deals with a right leg problem.
Either Paraguay or Japan awaits the winner in the quarterfinals.
