The Arsenal midfielder had only been on the pitch for 117 seconds after replacing Dani Olmo when he reacted quickest to fire home the decisive goal at SoFi Stadium in Los Angeles. It was Merino's second consecutive match-winning contribution from the bench, having also scored a stoppage-time winner against Portugal in the last 16.

Spain will now meet France in a blockbuster semi-final clash in Dallas on Tuesday, with Luis de la Fuente's side reaching the final four of the World Cup for only the second time in their history.

The quarter-final appeared destined for extra time after both sides traded goals in the first half, but Merino's opportunistic finish broke Belgium hearts.

Spain took control early on, with coach De la Fuente's decision to start Fabian Ruiz ahead of Pedri quickly paying off. After a flowing move involving Pedro Porro and teenage sensation Lamine Yamal, Dani Olmo's effort was saved by Thibaut Courtois, only for Ruiz to drill in the rebound after 30 minutes.

Belgium, however, refused to collapse despite being under pressure for long periods. They levelled four minutes before half-time when Timothy Castagne delivered an excellent cross and Charles De Ketelaere beat Pau Cubarsi to head past Unai Simon.

The goal ended Spain's remarkable defensive run at the tournament, with Simon finally beaten after more than 600 minutes without conceding.

Belgium entered the contest already weakened after captain Youri Tielemans was ruled out with an injury suffered during the warm-up, while Amadou Onana was also unavailable. Despite those setbacks, Rudi Garcia's side continued to threaten, with Kevin De Bruyne and Jeremy Doku creating problems for Spain's defence.

After the break, Spain continued to dominate possession but Belgium grew more dangerous on the counter-attack. Mikel Oyarzabal went close for Spain, while Courtois produced important saves to keep his side level.

The decisive moment arrived in the 71st minute when Courtois was forced off with what appeared to be a muscle injury. The Real Madrid goalkeeper left the field in tears and was replaced by Senne Lammens, making only his third international appearance.

The substitute goalkeeper's night ended in disappointment. In the 88th minute, Cubarsi's long-range effort lacked power but Lammens could not hold it, allowing Merino to pounce and smash the rebound into the roof of the net.

Belgium pushed for another equaliser in the closing stages, with Alexis Saelemaekers coming close to setting up Romelu Lukaku, but Spain held firm.

The defeat ended Belgium's World Cup campaign and added another painful chapter to the decline of their celebrated Golden Generation. Spain, meanwhile, extended their unbeaten run to 36 matches and moved one step closer to a second World Cup title.

Their reward is a semi-final showdown with France and Kylian Mbappe in what promises to be one of the tournament's biggest matches.

Super-sub Ollie Watkins' 90th-minute winner fires England into second successive Euro final