BOYDS, Md. -- D.C. United rode a strong first-half performance to a 2-1 victory against the Harrisburg City Islanders at the Maryland SoccerPlex on Tuesday, as goals from Boyzzz Khumalo and Andrew Jacobson proved to be just enough to hold off a spirited late challenge from the USL-2 underdogs in the quarterfinals of the U.S. Open Cup.Already down 2-0, Harrisburg looked to be cooked after the sending-off of captain Mo Oduor four minutes after halftime. But the visitors rallied gamely and came within inches of a stunning leveler on several occasions after Nicki Paterson's 64th-minute tally revived their hopes, leaving D.C. feeling lucky to advance despite their superior status in the U.S. professional pecking order.United will take on another lower-division side, the USL-1 Rochester Rhinos, in the semifinals on July 21, also at the Maryland SoccerPlex. The Rhinos defeated the USL-2 Wilmington Hammerheads 2-1 on Tuesday.United coach Tom Soehn once again rolled out a youthful starting lineup featuring several reserves, although he sprinkled in a bit of experience as Brazilian midfielder Fred made his return from an adductor injury and veteran right back Bryan Namoff captained the side.United came within inches of drawing first blood on a sixth-minute corner kick as Devon McTavish rose well to direct a header on target, but Harrisburg's Oduor cleared the bid off the goal line. The scoreless deadlock did not last much longer, however, as the home side pulled ahead just seconds later thanks to an athletic finish from Khumalo.Advancing down the United right, McTavish curled a low, angled cross from the right wing that skipped past Islanders defender Tim Velten, only to be met by Khumalo's lunging header at the far post as the South African gave D.C. a 1-0 lead.Islanders winger Ty Shipalane was showing some skillful touches along the right flank and he finally tested D.C. goalkeeper Milos Kocic with a speculative shot from long range, forcing the rookie netminder to make a fingertips save.But it was an isolated foray for the visitors and soon United doubled their advantage on a corner kick in the 18th minute. Fred curled an inswinging delivery into the danger area and Jacobson made the most of the space given to him inside the 6-yard box, bouncing a downward header off the turf and into the upper corner for his first goal of the season.Active and physical, United rookie Brandon Barklage was roaming the D.C. midfield with gusto but his evening ended prematurely after a clash along the sideline left him with a right knee injury. Soehn brought on left winger Rodney Wallace in his place, shifting Fred into a central role.Shipalane made the most of a quickly restarted free kick in the 37th minute, driving a shot towards the upper left corner of the United net only to be denied by an excellent leaping parry from Kocic.Khumalo was handed a golden chance to score again on the stroke of halftime, but he volleyed over the crossbar from close range and D.C. had to settle for a 2-0 lead at the break.The City Islanders came out of the locker room determined to impose themselves on the game more assertively and robust tackles were soon flying in from both sides. But tempers flared and referee Lee Suckle had to intervene when Oduor drove his elbow squarely into John DiRaimondo's face as the duo clashed at midfield in the 49th minute.Suckle immediately reached into his pocket to produce a straight red card for Harrisburg's captain while the prone DiRaimondo writhed in pain, receiving medical treatment before continuing.Seconds later McTavish provided yet another probing cross from the right, a booming delivery that dropped right into the path of Wallace's run to the far post. But finishing the chance required a tricky half-volley and the rookie's attempt flashed inches over the bar.United looked sure to score as Harrison was caught acres off his line as he chased a D.C. through ball, but after DiRaimondo centered into the box, neither Khumalo nor Ange N'Silu could produce a shot on the empty net as the crowd groaned.The City Islanders had been let off the hook and as is often the case, within minutes they managed to punish United's wastefulness with a goal of their own as play quickly switched to the other end.Harrisburg's initial counterattack was broken up by Greg Janicki but the big center back gave the ball back to Shipalane. His path to goal was blocked by Avery John but the loose ball eventually fell to Paterson, whose deflected shot flew past Kocic to cut the deficit in half and give hope to the shorthanded Islanders.Soehn moved to stifle the USL-2 side's momentum by bringing on playmaker Christian Gomez in place of Namoff, but it was Harrisburg who carved out the next chance. A crisp passing move released substitute Brandon Swartzendruber deep in the box and while Janicki did just enough to disrupt his run, the ball fell invitingly to David Schofield, who drilled a close-range shot that clanged off the underside of Kocic's crossbar and bounded away.The underdogs had their tails up and were now playing with far more pace and fluidity than they'd shown at full strength, pushing United onto the back foot as the MLS side found themselves sitting deep and looking to counter.N'Silu came close on a through ball that was narrowly cut out by Harrison and Wallace had a goal waved off for offsides, then Gomez and Fred combined for what looked to be the clinching goal in stoppage time -- but Harrisburg's lanky netminder made a sharp block of Gomez's blistering shot as his City Islanders stayed alive until the very end.Suckle's final whistle brought relief for the home fans as their team survived another late onslaught to advance to the tournament's semifinals, one step closer to successfully defending their title.D.C. United vs. Harrisburg City IslandersU.S. Open Cup QuarterfinalJuly 7, 2009 -- Maryland SoccerPlex Scoring Summary: DC -- Thabiso Khumalo (Devon McTavish) 8DC -- Andrew Jacobson (Fred) 18HCI -- Nicki Paterson (unassisted) 64D.C. United - Milos Kocic; Bryan Namoff (Christian Gomez 71), Greg Janicki, Avery John; Brandon Barklage (Rodney Wallace 29), Andrew Jacobson, Devon McTavish, John DiRaimondo, Fred; Thabiso Khumalo, Ange N'Silu. Substitutes not used: Josh Wicks, Marc Burch, Chris Pontius, Ben Olsen, Clyde SimmsHarrisburg - Chase Harrison, Steve Fisher (Brandon Swartzendruber 65), Geoff Bloes, Nicki Paterson, Tim Velten, Ty Shipalane, Nate Baker, Mo Oduor, Chad Severs (David Schofield 61), Jason Pelletier, Anthony Calvano (Kai Kasiguran 81). Substitutes not used: Tomer Chencinski, Cody Reinberg, Jason Massie, Phillip BuffingtonMisconduct Summary: HBC -- Mo Oduor (ejection; Serious Foul Play) 49DC -- Ange N'Silu (caution; Reckless Foul) 83DC -- Christian Gomez (caution; Tactical Foul) 86Referee: Lee SuckleReferee's Assistants: Mark Gorak; Terrence Andrews4th Official: Jim SadowskiTime of Game: 90:00Weather: Clear -and- 85 degreesAttendance: 2,137Charles Boehm is a contributor to dcunited.com.