Mikel Merino netted in stoppage time to send Spain into the World Cup quarter-finals with a cagey 1-0 win over Portugal on Monday as Cristiano Ronaldo said goodbye to the World Cup.

Merino expertly finished off fellow substitute Ferran Torres' through-ball a minute after the 90 as the 2010 World Cup winners and reigning European champions got the better of their neighbors.

It means 41-year-old Portugal great Ronaldo is left with Euro 2016 glory and two Nations League titles as his only international honors. He had previously said this would be his last World Cup.

The forward made his 27th and seemingly last World Cup appearance, having featured in a record-equalling six editions.

Spain have now gone a fifth game without letting in a goal at this edition and a record six across men's tournaments.

"We knew it was a match that would take time to settle," Spain captain Rodri said.

"They can cause damage on the counter-attack. I think we read the game perfectly. I’d like to single out Mikel Merino - it’s not easy to come on in those minutes and he’s been decisive once again."

In Friday's quarter-final near Los Angeles, the Spanish will play Belgium, who beat co-hosts United States 4-1 in the late game.

Portugal coach Roberto Martinez, a Spaniard, proudly sang the Portuguese anthem before kick-off as the two Iberian rivals - both 2030 co-hosts - locked horns for the 42nd time.

Martinez confirmed he is leaving the job having failed to become the first coach to beat his country of birth at a men's global tournament.

"We can be proud of the players for their performance. We showed heart and passion," Martinez said. "Luck was not on our side, it was an even game."

Later he told a news conference: "It is definitely my last match for Portugal." Jorge Jesus is widely expected to take over.

The coach brought back João Félix for Rafael Leao while Spain were as expected.

In-form Spain forward Mikel Oyarzabal missed a gilt-edge chance early on at Cowboys Stadium near Dallas and Ronaldo saw a snap-shot saved by Unai Simon.

Lamine Yamal and Alex Baena soon tested Diogo Costa at the other end before Ronaldo went close with a flick and team-mate Nuno Mendes hit the bar via a deflection.

The second half was a slower affair and Spain edged it, but extra-time looked certain until Arsenal midfielder Merino struck just six minutes after coming on.

Bernardo Silva headed over for Portugal later in injury time with a good opportunity.

It led to emotional scenes from a tearful Ronaldo as he saluted the crowd at full-time.

In the previous 3-0 win over Austria, Spain keeper Simon reached 519 minutes without conceding at the World Cup, breaking Walter Zenga's record for Italy. He has now extended the mark against the record international men's scorer of all time.