Ivan Alonso made it five goals in four games with a brace against Sud America, converting twice in the space of five minutes at the Estadio Centenario.
Alonso slotted a penalty in the 62nd minute and then dinked the ball inside the back post in the 66th, although it looked as if the 34-year-old striker was attempting a cross.
Fenix (nine points, plus seven goal difference) remain top of the Primera Division table after a 3-0 win at Racing, with Nacional (nine points, plus six) close behind.
Maximiliano Perez, Franco Acosta and Anibal Hernandez scored the goals for Fenix as they raced out to a three-goal lead by half-time and then held on in the second half at Racing's Parque Osvaldo Roberto.
Wanderers thrashed Cerro Largo 4-0 thanks to a brace from Rodrigo Pastorini to claim third in the standings on seven points, while El Tanque Sisley (six points) sit fourth following a 1-0 win over Miramar Misiones.
Juventud remain pointless through three rounds after a 2-1 loss at Defensor Sporting, while Danubio defeated Liverpool 2-1.
River Plate were 1-0 up at home to Cerro before their clash was suspended.
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