BOYDS, Md. -- Despite the fact that their unheralded adversaries were the last amateur side left in the competition, D.C. United had to dig deep -- and ride their luck -- to secure a hard-fought 2-0 win over the Premier Development League's Ocean City Barons in U.S. Open Cup third-round action on Tuesday night.United will now move on to continue their defense of the Dewar Trophy against the Harrisburg City Islanders next month, but those in attendance at the Maryland SoccerPlex will know how easily the D.C. could have found themselves knocked out of nation's oldest soccer tournament.On a night when their MLS counterparts from Chicago and New England crashed out at the hands of minor-league upstarts, United veered dangerously close to the same fate as their lack of sharpness around goal played into the Barons' game plan and set up a tense finish.United fielded several reserves in their first XI and it showed in the team's first-half performance. While the Black-and-Red enjoyed the lion's share of possession, they struggled to break down Ocean City's deep-lying 4-5-1 shape and even found themselves caught on the counterattack on several occasions."For them, it's a big game and they came out and competed hard. Defensively I knew they were going to tuck in, and they did, and looked for us to make a mistake, and we almost did in the first half," said D.C. head coach Tom Soehn. "At the end they gave it a push. So you credit them for competing."The average age of Ocean City's starting lineup against D.C. was slightly more than 21 years, but the Barons displayed the shrewdness of a veteran side as they adhered to a defend-and-counter approach that frustrated their top-tier opponents. With dogged defending and disciplined tactics, the PDL side showed how they knocked off professional sides Crystal Palace Baltimore and Real Maryland in the two previous rounds of the Open Cup."Really mature," said D.C. goalkeeper Milos Kocic of Ocean City's gritty display. "A pretty young team of college guys -- I'm really surprised, it was great defending and they were waiting for their chances, playing on the counterattack. It was really mature, really good defending."As Barons goalkeeper Tunde Ogunbiyi repeatedly stymied United and time ticked away with the score stuck at 0-0, Soehn was left with no choice but to pull first-choice attackers Chris Pontius and Christian Gomez off the bench. The duo soon made their influence felt with a bevy of scoring chances that eventually culminated with Gomez's decisive penalty kick in the 74th minute."You always want to play -- even in practice you want to be starting," said the Argentinean. "But obviously the coach went with other players, giving them an opportunity, and I was just ready when he called upon me, and we were able to get the result."Kocic was left a spectator for long stretches of the match, only to be called upon for several spectacular saves as the Barons finally threw numbers forward in the final minutes - and even at that, Ocean City saw a near-equalizer clang off the bottom of the crossbar in a frenzied sequence just before full time."[Ocean City] had to open up and for me as a 'keeper, I have to be focused every 90 minutes," said the Serbian-born netminder. "A lot of shots, a lot of excitement in front of my goal, so the hardest thing for me was to stay focused, because this team, you see when they open up and start to play they're really dangerous."