Leading by four points prior to kick-off Jorge Jesus' men suffered a surprise 2-1 defeat, but Porto missed the chance to cut the deficit to just one point as Nacional came from behind to earn a 1-1 draw.
An early goal from Eduardo Salvio appeared to have set Benfica – who saw Luisao sent off late in the game - on their way to a sixth-successive league victory, however, they fell apart after the break as Ukra and Yonathan Del Valle snatched a memorable win for the hosts.
Like their rivals, Porto seemed to be on course for victory against Nacional when Cristian Tello fired home his seventh goal of the season from the edge of the penalty area on the stroke of half time.
They were unable to secure the win that would have seen them move to within a point of the leaders though, as Wagner earned Nacional a point two minutes after the hour with a close-range finish.
Meanwhile, at the bottom Penafiel looked on course to boost their survival hopes against Estoril as they opened up a 3-1 lead, but Leo Bonatini and Ruben Fernandes ensured the points were shared in a thrilling 3-3 draw.
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