Porto were held to a scoreless stalemate away to Sporting Lisbon, which gives Benfica a chance to top the Liga Sagres table on Sunday.
Vitor Pereira's men couldn't account for the 11th-placed Sporting - who moved to 10th courtesy of their point gained.
The point for Porto may not spare them dropping to second by the time the weekend's out, though, as they lead by that margin over Benfica, who are away to Beira-Mar on Sunday.
It was just the fifth time in 21 matches this season Porto have failed to win.
Sporting held on for the draw despite defender Marcos Rojo being dismissed in the 77th minute at the Estadio Jose Alverde.
In the day's other clash, Cicero struck a brace to help Pacos de Ferreira beat Vitoria Setubal 2-0.
Pacos consolidated third spot in the league, but still trail leaders Porto by 15 points.