Cerro Porteno defeated 3 de Febrero 2-1 to move to six points after two rounds of the Division Profesional's Apertura season but Rubio Nu kept the pressure on the leaders with a crushing victory over their city rivals.
Rubio Nu have four points and are just ahead of third-placed Libertad on goal difference after the latter drew 0-0 with Sportivo Luqueno.
Jorge Ortega started Rubio Nu's rout of Nacional in the 19th minute, bursting onto a long ball before striking a half-volley from the edge of the area that clipped the post on its way into the net.
A 55th-minute penalty from Silvio Torales saw Nacional equalise but the visitors had no response to Rubio Nu's three second-half goals.
Ortega gave the hosts the lead again in the 58th minute, heading home from a free-kick, while Claudio Correa made it 3-1 eight minutes later with a sharp finish at the back post after a cross from the left.
Francisco Gomez struck the home side's final goal, thrashing the ball into the net after Nacional's goalkeeper had blocked the midfielder's initial shot.
The victory continued Rubio Nu's recent dominance of Nacional - four games without a loss, including three wins.
On Friday, Cerro Porteno made it two wins from as many games with Daniel Guiza scoring the decisive goal against 3 de Febrero.
After Oscar Romero had given Cerro Porteno a 1-0 lead at half-time, 3 de Febrero equalised just before the hour-mark when Juan Pablo Raponi expertly chipped the home side's goalkeeper Roberto Fernandez.
But Cerro Porteno struck back just five minutes later with Guiza getting his head to a cross from the left wing, putting the hosts on their way to three points.
In other results, Guarani defeated Olimpia 2-1, Deportivo Capiata drew 0-0 with Sol de America and General Diaz came from behind to claim a 1-1 draw at 12 de Octubre.
