Cristiano Ronaldo's World Cup dream ended in tears as Mikel Merino stepped off the bench to fire Spain into the quarterfinals with an injury-time winner in Dallas.

The Portuguese superstar had been a passenger for much of a low-key contest that belatedly sprang into life when Merino followed up his quick free quick to nip into the box and fire the only goal of the game past keeper Diogo Costa.

Spain had held the upper hand throughout but they were still forced to hold off a frantic late push from the Portuguese with Bernardo Silva lofting a header narrowly over bar from Sergio Conceicao's right-wing cross.

Ultimately Portugal paid the price for playing it too cagey for much of the second half, with Arsenal man Merino capitalising with a clever move to send his country through to the last eight.

Mikel Oyarzabal was guilty of a shocking miss for the Spaniards with eight minutes left on the clock when he was put through by Dani Olmo but appeared to think he was offside and casually dragged his shot wide to the right of goal.

Spain had another great chance in the 16th minute when they forced a brilliant double save from Costa, first an effort from Lamine Yamal after cutting in from the right and moments later a rebound from Álex Baena that was tipped around the post.

Ronaldo made little impression in the opening period but could still have made the decisive move when he clambered to his feet after being pulled over by Rodri in the box and moments later poked a shot from a tight angle that Spain keeper Unai Simón was forced to parry.

And Spain rode their luck four minutes before the break as they were caught sleeping at a set-play and Nuno Mendes's strike took a heavy deflection off Pedro Porro before ricocheting out to safety off the bar.

A slow-building second period came alive on the hour mark when Portugal gave the ball away again their own half and Dani Olmo cued up Pedri, whose goalbound effort from the edge of the box was deflected over the bar by Rentao Veiga.

Baena drove a low shot that was easily saved, and Yamal whipped in a free-kick from the right that was touched over the bar by Costa, but otherwise, the Portuguese were doing a good job of nullifying Spain's attacking threat.

Spain cranked up the pressure, with Dani Olmo coming close then Ferran Torres playing havoc with a low ball across the six-yard box before Merino's late effort almost certainly brought Ronaldo's World Cup career to a disappointing close.