Sporting Lisbon missed out on the chance to go two points clear at the top of the Primeira Liga in a goalless draw with Nacional on Saturday.
Victory would have seen the men from the Portuguese capital put daylight between themselves and second-placed Porto, but they will have to settle for being part of a trio of clubs - with Benfica - tied on 33 points heading into the mid-season break.
The hosts will have every reason to feel aggrieved that they did not come away from the clash with maximum points.
Sporting substitute Islam Slimani had the ball in the back of the net midway through the second half, but had the effort harshly ruled out by referee Manuel Mota for an alleged push on visiting goalkeeper Eduardo Gottardi.
Sporting's players surrounded the referee at the final whistle, with some players having to be restrained as tensions boiled over.
Elsewhere, Vitoria Guimaraes climbed to fifth with a 3-0 victory over struggling Academica, who have now gone three matches without a win.
Marco Matias opened the scoring for the hosts in the 27th minute at Estadio Dom Afonso Henriques, before Jean Barrientos and Andre Andre put the game to bed in the second half.
Guimaraes' task was made all the easier by the dismissal of Rafik Halliche shortly after the half-hour mark.
Pacos de Ferreira failed to climb out of the relegation zone as they played out a goalless draw with Rio Ave in the day's other match.
The draw does, however, bring to an end Pacos' three-match league losing streak.