De Rosario became the third DC United player to score a hat trick in Open Cup play, following in the footsteps of Raul Diaz Arce (2000) and Abdul Thompson Conteh (2001).
His squad moves on to the quarterfinals where they will host the New England Revolution.
The fourth round matchup was a rematch from last year’s tournament when the sides met in the same round and location, with the Union prevailing.
Desperate for success due to its worst start in team history, DC United came out with a sense of urgency that Philadelphia could not match. The hosts won nearly every 50-50 ball and forced the Union into poor passes and decisions.
It came as no surprise then that DC had the first chance five minutes in.
Chris Pontius took advantage of lackadaisical defending to shoot in a cross from the left corner. His pass found a wide open Nick DeLeon, who missed the near post by inches.
DeLeon made up for his miss in the 24th minute by taking a shot from the top of the Philadelphia box. Amobi Okugo deflected the shot right into the path of De Rosario, who sent a left-footed curler past the out-stretched arms of Zac MacMath.
United saw out the half comfortably bar one tough injury to Chris Pontius.
After taking an elbow from Sheanon Williams, Pontius got fouled on a strong challenge from Okugo — a challenge that earned him a yellow card.
Pontius was forced out to be replaced by Sainey Nyassi.
The Union pressed forward more in the second half, but could only put together a few half-chances.
However, it would ultimately be defensive lapses that determined the outcome.
In the 75th minute, Amobi Okugo gave the ball away to Lionard Pajoy under very little pressure. Pajoy passed to De Rosario, who calmly slid it past the Union’s young keeper.
Just 40 seconds later United had a lapse of its own.
Philadelphia substitute Don Anding blazed down the left side and passed to MLS’s leading goal-scorer Jack McInerney.
DC failed to attack so McInerney curled a right-footed shot into the bottom-right corner of the net to cut the lead in half.
Another careless Union mistake killed the game in the 85th minute.
Well outside his box, MacMath passed a free kick to Okugo, whose touch went straight to De Rosario.
The Canadian international took a few steps and put the ball into an empty net from about 30 yards away to make it 3-1.
DC United 3:1 Philadelphia Union
Maryland SoccerPlex – Boyds, Md.
DCU: Dwayne De Rosario (Nick DeLeon) 24
DCU: Dwayne De Rosario (Lionard Pajoy) 75
PU: Jack McInerney (Don Anding) 76
DCU: Dwayne De Rosario (unassisted) 85
DC United: Joe Willis; James Riley (Brandon McDonald-62), Ethan White (Lionard Pajoy-67), Daniel Woolard, Taylor Kemp; Nick DeLeon, Perry Kitchen, John Thorrington, Chris Pontius (Sainey Nyassi-46); Dwayne De Rosario, Casey Townsend
Philadelphia Union: Zac MacMath; Raymon Gaddis, Jeff Parke, Amobi Okugo, Sheanon Williams; Michael Farfan (Don Anding-72), Brian Carroll, Keon Daniel (Leo Fernandes-46), Sebastian Le Toux; Conor Casey (Antoine Hoppenot-65), Jack McInerney
PU: Amobi Okugo (caution – 45+)
PU: Leo Fernandes (caution – 56)