Fourth-placed Estoril looked set to claim a first win in four Primeira Liga matches courtesy of Seba's first-half strike.
But Evandro then received a straight red card for an over-the-ball tackle in the 69th minute and Arouca levelled seven minutes later through a header from Roberto.
Estoril now sit nine points behind Porto and could be overhauled by either Vitoria Guimaraes or Nacional, who meet on Monday, while Arouca are four points above the relegation zone.
Brazilian forward Seba opened the scoring nine minutes before the break, lofting the ball over Cassio after Arouca's goalkeeper had raced out of his area.
Arouca were given renewed hope by Evandro's dismissal, however, which came during a feisty second half featuring seven yellow cards.
The visitors levelled when Roberto met a cross from the right with a downward header into the bottom-left corner.
