Arsenal midfielder Mikel Merino was the late hero for Spain as La Roja waited until injury time to break the deadlock with a defence-splitting pass from Ferran Torres and a composed finish from Merino.
In defence, Marc Cucurella remained an ever-present feature of this Spain side under Luis de la Fuente, with another full 90 minutes under his belt at left-back.
He was tasked with the challenge of handling Chelsea winger Pedro Neto, and was required defensively to support with the handling of Cristiano
Ronaldo through the middle in a game of few chances for either side.
Spain boasted the best chances, accumulating 1.77 xG over 15 shots, but two thirds of those came from outside the box. Portugal defended well, and also pinned Cucurella back effectively down the left flank.
Unlike in previous games, his offensive contributions were limited, with no chances created and no shots, although he did complete three of four crosses.
Instead his workload came defensively, with five recoveries and seven clearances, especially amid an onslaught from Portugal late on with new Real Madrid signing Bernardo Silva in the thick of the action with a late headed chance to equalise.
Spain will now face either the host nation the United States or Belgium in the quarter-final in Los Angeles on Friday. The winner would then face France or Morocco back in Dallas next Tuesday to fight for a place in the final.
Defeat for Portugal means international retirement is imminent for Real Madrid legend Cristiano Ronaldo, while coach Roberto Martinez is also set to depart. Bernardo Silva, benched for this game and used as an 83rd minute substitute, may also have played his last World Cup game for his country.
