The World Cup's A-list attackers have been dominating the headlines this summer but it is the Spanish backline which is proving the most extraordinary of the lot.Luis de la Fuente's water-tight La Roja outfit are into the quarterfinals after edging out Iberian neighbors Portugal in a true chess match at Dallas Stadium, where substitute Mikel Merino netted the game's only goal in the first minute of stoppage time.Spain's remarkable defense, which had only allowed three shots on target prior to Monday's game, is still yet to concede a goal after five games. They also march into the next round, where they will meet either USA or Belgium, on a record run of six straight clean sheets at this famous tournament.In a game of very few clear-cut opportunities, Cristiano Ronaldo left the World Cup stage for the last time on the most depressing of notes. Sporting greats rarely go out on a high, often the result of outstaying their welcome, and that proved the case here.Portugal's faded force touched the ball just 19 times in 96 minutes, the fewest of any starting player by some distance, before shedding tears after his country's elimination signaled the end of his dream to win one of the only trophies that has eluded him in his illustrious career. And the biggest one at that.After Goncalo Ramos' dramatic winner against Croatia in the last 32, Portugal boss Roberto Martinez had an unenviable decision to make at the pinpoint of his attack: either unleash his fresher, more in-form striker - who was averaging a goal or assist every 37 minutes at World Cups heading into Monday's game - or stick with Portugal's captain, their national icon, for what could be one last rodeo in Texas.