Porto lost 2-0 to Belenenses to remain at 70 points, one fewer than first-place Benfica in the race for the Primeira Liga championship.
The result on Monday makes it likely that the title will be decided when Benfica and Porto face each other later this month in the clasico, rpeorts Efe.
The home side, meanwhile, climb to 11th place with 32 points from 28 matches, nine points above the drop zone.
Porto fell behind in the 10th minute when a breakdown in communication between defenders Felipe and Yordan Osorio - making his first start with the team - left Belenenses midfielder Nathan alone in front of goal.
The Brazilian deftly lifted the ball over Porto keeper Iker Casillas to make it 1-0 for the hosts.
Porto returned from the break determined to pull level, but Belenenses keeper Andre Moreira was solid, stopping shots by Ricardo Pereira, Felipe and Gonçalo Paciência.
With 20 minutes left in regulation, Maurides got his head to Fredy's free kick and beat Casillas to bring the advantage to 2-0 and Belenenses hung on to become only the second team to defeat Porto in league play this season.
For Casillas, the long-time Real Madrid starter and Spain captain, it was a disappointing outcome for his 1,000th professional match.
--IANS
gau/mr
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