Merino netted in the 88th minute as the European champions beat the injury-hit Belgians 2-1 to set up a blockbuster with France for a place in the final.
Only four days ago Merino scored a clutch goal as a substitute in injury time to beat Portugal.
"I've done this again, and it's happened to me again, so it would seem that coincidence exists," a smiling Merino said. "If you're ready and you try, it can happen for you. I'm very, very pleased."
Fabián Ruiz had given Spain the lead on 30 minutes before Charles De Ketelaere equalised, meaning Unai Simón conceded for the first time in a record 649 World Cup minutes spread over two tournaments.
Merino, just like in the last-16 win over Portugal, then came off the bench and scored after a mistake from Belgium reserve keeper Senne Lammens.
He has scored two goals in his first World Cup - and they're both historic.
"Honestly, it's crazy to be able to help the team once again," he said. "This time in a different way, but at the same time to believe and trust that the opposing goalkeeper could make a mistake and to stay alert.
" ... I prepare for when the moment comes, and hopefully they keep coming."
The mouth-watering meeting with the French near Dallas on Tuesday will be a repeat of the Euro 2024 semi won by La Roja, whose only World Cup title was in 2010 in South Africa.
France, 1998 and 2018 winners who were runners-up to Argentina four years ago, know they will face their most difficult opponents so far after Spain dominated a Belgian side shorn of two key midfielders in Los Angeles.
"It will be a clash of giants," Spain coach Luis De La Fuente said through a translator.
Belgium were hit by captain Youri Tielemans having to pull out of the game during the warm-up with an apparent hamstring problem, adding to Amadou Onana's absence after a knee ligament tear.
An injury to a tearful Thibaut Courtois in the 71st minute led to Manchester United's Lammens entering the fray in the Belgium goal.
Both sides pushed in an absorbing second half but Spain managed to exploit that after bringing on Nico Williams and Merino.
Lammens spilled a long-range effort from Pau Cubarsi and Arsenal's Merino did the rest to determine the outcome of the tie.
"I gave him a big hug, much more I cannot do in the moment," Courtois, who made four saves in the match, said. "He's a great goalkeeper. He will only get stronger from this."
Belgium coach Garcia bemoaned all his injuries, saying Kevin De Bruyne was also not fit.
"That was naturally tough for us," said Garcia. "Sometimes details in football count. But I'm still proud of my players, they produced a strong performance."
The result left Spain unbeaten in 37 straight competitive matches since March, 2023, while ending Belgium's streak of 18 consecutive unbeaten matches across all competitions.
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