Spain have defeated Portugal 1-0 in stoppage time in the Round of 16 to secure a place in the World Cup quarterfinals.
Merino broke the deadlock in the first minute of added time at AT&T Stadium outside Dallas to send the Spaniards into a last-eight clash against either Belgium or the United States in Los Angeles on Friday.
In front of an expectant crowd of 70,649 at the air-conditioned home of the Dallas Cowboys, a match that on paper promised so much failed to really deliver.
Extra time looked certain before fellow substitute Ferran Torres slipped in Merino to settle the contest in the first minute of injury time.
It meant a sour final World Cup match for the 41-year-old Ronaldo, now playing in Saudi Arabia, having confirmed on the eve of the encounter that this would be his last dance on football's biggest stage.
The neighbours came into the contest both unbeaten, but while Portugal had not been entirely convincing, Spain had not even conceded a goal in the tournament.
One of the pre-tournament favourites, they still haven't.
For all their pedigree, talent and history, Spain have only lifted the World Cup once, in 2010, while Portugal's best was third place in 1966.
In Ronaldo and Spain's 18-year-old Barcelona prodigy Lamine Yamal, the last-16 showdown was touted as a showcase of two extraordinary talents at opposite ends of their careers.
Ronaldo cheered, jeered
Spain should have been ahead with less than 10 minutes gone when Dani Olmo played in Mikel Oyarzabal, but with only goalkeeper Diogo Costa to beat, he somehow pulled it well wide.
Portugal skipper Ronaldo had his first chance to add to his three goals in North America, stinging the palms of goalkeeper Unai Simon from a tight angle.
Luis de la Fuente's Spain cranked up the pressure, Yamal and Alex Baena testing Costa twice in quick succession, the second a wonderful fingertip save.
Ronaldo, jeered and cheered in equal measure, was a marginal presence in the central attacking role he now occupies in the twilight of his career.
Portugal ended the half on top, and Roberto Martinez's side were inches from taking the lead when Spain full-back Pedro Porro stuck out his head and directed a Nuno Mendes shot onto his own crossbar.
The winger Yamal had endured a frustrating time up against Mendes, but the Paris Saint-Germain fullback was forced off with an injury after 56 minutes, a blow to Portugal.
The game needed a spark and Ronaldo, well known to Spain from his time at Real Madrid, threw up his arms when a pass went astray.
Martinez made double changes on 71 and 83 minutes, but kept Ronaldo on.
And in a cagey game, Arsenal midfielder Merino won it at the death with a composed finish that was out of keeping with much of what had gone on before.
END/TRT WORLD/ASA
