Dario Franco's men made it back-to-back victories to move to 16th with 18 points through 16 matches - just six points behind seventh-placed Newell's.
Miguel Bertocchi was the provider of the winning goal at the Estadio Norberto Tito Tomaghello, releasing Brian Fernandez with a ball through played from inside his own defensive half.
Fernandez did the rest, beating Newell's goalkeeper Oscar Ustari in a one-on-one to give the hosts the lead in the 71st minute.
Gustavo Raggio's visitors would have their chances to equalise, with Defensa down to 10 men four minutes after the goal when Javier Yacuzzi earned his second yellow card of the game.
Newell's best chance to capitalise on their numerical advantage came in the 84th minute, when Ciro Rius fouled former Sunderland attacker Ignacio Scocco in the area.
Scocco stepped up to take the spot kick, but Defensa keeper Diego Pellegrino kept out the effort to help his side to another upset victory.