Spain vs Portugal is always massive, but with a World Cup quarter-final spot on the line, it’s going to be absolute chess. Photo: AFP

Spain and Portugal renew one of international football's fiercest rivalries on Monday night with a place in the World Cup quarter-finals at stake.

Spain have gathered momentum as the tournament has progressed, scoring eight goals without conceding in four matches. Portugal have matched that attacking output with eight goals of their own while conceding just twice.

Here are five key match-ups that could shape the outcome of what promises to be one of the standout ties of the Round of 16 at Dallas Stadium.

The headline duel pits the tournament's brightest young attacker against arguably its best left-back. Yamal has grown into the World Cup after returning from injury, producing influential displays against Saudi Arabia and Austria with his fearless dribbling and creativity. Mendes, meanwhile, has been one of Portugal's most consistent performers, combining defensive discipline with the pace to launch dangerous counter-attacks. If he can stop Yamal cutting inside onto his favoured left foot, Portugal will go a long way towards blunting Spain's most dangerous weapon.