Porto's slim Primeira Liga title chances were dealt another blow on Sunday as they lost a two-goal lead to be held 2-2 by Vitoria Guimaraes.
The champions opened up an advantage thanks to first-half strikes from Ricardo Quaresma and Lica, before the hosts hit back.
They responded through Moussa Maazou and Marco Matias either side of half-time to snatch a share of the points.
The result leaves Porto nine points adrift of leaders and bitter rivals Benfica and without a win in their last four outings, heaping yet more pressure on coach Paulo Fonseca.
Benfica managed to re-establish their lead at the top of the table to five points thanks to 1-0 win away to Belenenses, which came courtesy of a Nicolas Gaitan goal within the first 10 minutes.
The win was Benfica's fourth in a row in the league, with Jorge Jesus' side seemingly strengthening their grip on a 33rd league title as each week passes.
Elsewhere, bottom-of-the-table Pacos de Ferreira came from behind to secure a vital win at home to Maritimo.
The visitors took the lead just after the half-hour mark thorough Derley before Jean Michael Seri levelled and then Flavio Boaventura gave his side the lead with just seven minutes to go.
It got even better for the hosts just two minutes later when Bebe wrapped up the win with a third.
Elsewhere, Gil Vicente eased their own relegation concerns by ending an 11-game league winless run thanks to a Cesar Peixoto own goal at home to Vitoria Setubal, while a goal from Deigo Barcellos rescued a point for Nacional at home to Rio Ave.