New signing Walter Pandiani opened the scoring for Miramar with a penalty, while Augustin Miraballes and Ignacio Christophersen added goals after half-time to wrap up the visitors' victory on Tuesday.
The opening round win saw Miramar move into equal second with Liverpool and Wanderers after the latter two triumphed on Saturday and Sunday respectively.
Liverpool had to come from behind to overcome Defensor Sporting 3-1, while a brace from Gaston Rodriguez saw Wanderers triumph by the same scoreline against Cerro.
At Rentistas' stadium in Montevideo, Pandiani - the former Deportivo La Coruna, Birmingham City and Osasuna striker - put Miramar ahead after 24 minutes.
It was 2-0 in the 49th minute when Miraballes struck and, while Octavio Rivero scored for the home side with 29 minutes remaining, Christophersen wrapped up the three points for Miramar with his goal soon after.
In the other game on Tuesday, Danubio and Cerro Largo drew 1-1.
Sud America lead the Primera Division's Clausura campaign on goal difference after thrashing Juventud 3-0 on the opening day of the season.
Gonzalo Malan gave Sud America the lead in the 22nd minute with a header from a cross and the hosts maintained their 1-0 lead at half-time despite both teams having a player sent off in the first half.
Juventud's Diego Rodriguez was given his marching orders in the 38th minute, while Antonio Fernandez of Sud America copped a second yellow card three minutes before the break.
Santiago Biglieri doubled the home side's advantage from the penalty spot just after the hour mark, following a foul by Christian Latorre, which saw the Juventud forward red carded, while Nicolas Royon capped off Sud America's win in the 88th minute with a sharp finish.
In other results, Penarol needed a late comeback to overcome River Plate 2-1, Nacional trumped Racing 2-0 and Fenix defeated El Tanque Sisley 2-1.
