Benfica and Porto both won on their travels in Sunday's Primeira Liga action and Nacional's unbeaten run continued.
Jorge Jesus' men remain four points clear at the top of the Portuguese top flight after a 2-0 win at Pacos de Ferreira.
Benfica dominated possession but were not at their fluent best, with scores locked level at the break.
Argentine defender Ezequiel Garay broke the deadlock in the 54th minute though, converting Ruben Amorim's cross with aplomb.
Garay was unmarked and did not waste his free header at the far post, firing a bullet header beyond Pacos goalkeeper Matias Degra.
And 14 minutes later Serbian Lazar Markovic sealed the result, running onto a loose ball and racing clear.
Markovic did not break stride before thrashing past Degra from an acute angle, as Benfica extended their Primeira Liga unbeaten streak to 18 matches.
Porto won 2-1 at Gil Vicente as Silvestre Varela continued his impressive 2014.
The attacker, who has scored six goals this year for his club, netted both of Porto's goals on this occasion.
He opened the scoring in the 18th minute when he beat Gabriel Moura to a ball to head home at the far post.
His second – which came in the 53rd minute – seemed to awaken the hosts, who responded almost immediately through Hugo Vieira.
But Porto hung on for a win that sees them keep the pressure on Benfica.
The fourth-placed Nacional are now unbeaten in their last seven matches but will feel aggrieved they could not beat Arouca in a 1-1 draw.
Claro's early goal fired Arouca in front but Fernando Marcal's leveller just after the hour-mark buoyed Nacional.
They could not find a winner against Arouca, who sit 12th, though, meaning they are now nine points behind Sporting Lisbon.
Other results on Sunday saw Nuno Rocha's goal give Maritimo a 1-0 home win against Vitoria Setubal and Vitoria Guimaraes also won 1-0, against Rio Ave.