The European champions secured their place in the last eight after Merino calmly finished in the first minute of added time, breaking Portuguese hearts in a tense contest at the AT&T Stadium in Arlington, Texas. Spain will now face Belgium in Friday's quarter-final after the Red Devils thrashed the United States 4-1 earlier on Monday. Despite the star-studded line-ups and a crowd of 70,649, the Iberian showdown struggled to live up to expectations for much of the match, with both sides cancelling each other out before Merino's late intervention. The game also marked the final World Cup appearance of Portugal captain Cristiano Ronaldo. The 41-year-old, who confirmed before the match that the tournament would be his last, was unable to inspire his side as his long pursuit of a World Cup title came to an emotional end. Spain, who arrived in the knockout stage unbeaten and yet to concede a goal, maintained their defensive record while extending their impressive tournament run. Before kick-off, the stadium observed a poignant moment as an image of the late Portugal forward Diogo Jota was displayed on the giant screen. Spain nearly took an early lead when Dani Olmo sent Mikel Oyarzabal through on goal, but the forward dragged his effort wide with only goalkeeper Diogo Costa to beat. Ronaldo had Portugal's best chance of the opening half, forcing Unai Simon into a save from a tight angle, while Costa produced excellent stops to deny Lamine Yamal and Alex Baena at the other end. Portugal came closest to breaking the deadlock before halftime when Pedro Porro inadvertently headed Nuno Mendes' effort onto his own crossbar. The second half remained cagey, with Portugal suffering a setback when impressive full-back Nuno Mendes was forced off through injury. As extra time beckoned, Spain finally found the breakthrough. Fellow substitute Ferran Torres slipped a perfectly weighted pass into the path of Merino, who kept his composure to slot past Costa and send the European champions into the quarter-finals. The late winner ensured a heartbreaking farewell for Ronaldo, whose sixth and final World Cup appearance ended without the trophy that had eluded him throughout his legendary career.