With only four minutes played, Damien Perrinelle's cross found Bradley Wright-Phillips and the former Southampton forward buried his header past a helpless Blake as New York looked to be running away with proceedings.
With Philadelphia struggling to put together any sort of a passing game, the Red Bulls netted a third on 17 minutes as Grella scored his ninth of the campaign to all but seal victory in the opening stages.
Union's nightmare first half was rounded off when Kemar Lawrence scored New York's fourth on 44 minutes as the visitors' defence failed to contain the home side in attack.
However, New York were able to hold on move six clear of DC United, who beat Chicago Fire to move into second place. body check tags ::