"I couldn't have imagined something like this even in my wildest dreams - another goal in the dying minutes," said Merino, who had already scored in stoppage time in Spain's 1-0 win over Portugal on Monday.
"I thought it would take a long time to repeat a feat like that... and I managed it in the very next game!"
Late in the game and with the score tied 1-1, Spain's Pau Cubarsi had shot low from range.
Belgian substitute 'keeper Senne Lammens should have held onto the ball. Instead it bounced toward Merino, who had only been on the pitch two minutes himself. The Arsenal player was first to the rebound and fired it in.
Merino said it was not purely a coincidence, attributing his dramatic late success to "going out there with drive" and belief in himself to "contribute wherever you're needed."
"That's exactly what I did," he said.
Merino said his team's run to a semi-final clash with France at the World Cup has so far been "less flashy" than Spain's victorious campaign at the European Championships two years ago.
"It was a struggle today," he admitted.
The win against Belgium sets up a tough test against France, who are the World Cup favorites, but who Spain beat 2-1 at the Euros.
"It's going to be a head-to-head battle. That's how semifinals always are. We're going into the match against France hungry and eager," he said.
"It's an elite-level match, the highest level. We are two teams playing at the very top of our game.
"We'll try to do our absolute best," he said.
World Cup 2026
The 2026 World Cup will be held from June 11th to July 19th in the United States, Canada and Mexico. The tournament will feature 48 national teams and will be played in 16 modern stadiums.
Match schedule and times | Draw | England at the World Cup | Bellingham: England 'shouldn't fear anyone' | How to watch the World Cup | Predictions and odds
