The Dragons have 42 points after 16 matches, while archrivals Sporting are fourth with 34 points, reports Efe.
Yacine Brahimi opened the scoring for Porto in the 32nd minute and Tiquinho Soares made it 2-0 for the hosts six minutes later. But Nacional reacted and clawed back a goal in the 40th minute courtesy of a strike by Bryan Rochez.
Brahimi got a second in the 57th minute to assure the victory.
Earlier Monday in Tondela, the home side prevailed 2-1 over Sporting despite having to play the final for 40 minutes with only 10 men after João Jaquite was sent off with a straight red card for a foul against Nani.
Tondela jumped out to the lead with Juan Delgado's goal in the 6th minute. Tomane boosted the advantage to 2-0 in the 74th minute.
Moreirense won 1-0 at home to Aves to climb to 5th place with 28 points.
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