BOYDS, Md. -- In the last two rounds of U.S. Open Cup play, D.C. United have produced quality soccer for long periods only to let lower-tier opponents linger in contention until the very end due to poor finishing. That pattern continued at the Maryland SoccerPlex on Tuesday night as the Black-and-Red failed to put away the Rochester Rhinos after outplaying the USL-1 side for most of the match. But despite that flirtation with disaster, United did enough to win 2-1 as a Jaime Moreno penalty kick and a late winner from Boyzzz Khumalo outweighed a scrappy free kick goal from Rhinos striker Taiwo Atieno to send D.C. into the Open Cup final in six weeks' time.United's first XI featured a blend of reserves and regulars as Jaime Moreno, Avery John and Rodney Wallace were the only additions to the group that defeated Harrisburg in the quarterfinal round.The semifinal began cagily as both sides tussled in midfield with few clear chances until the 12th minute, when a sustained D.C. passing sequence led to Devon McTavish skipping a low cross through the Rhinos goalmouth that no one in a black jersey could finish off.Five minutes later Rochester goalkeeper Tim Mella nearly notched an assist off a booming clearance that flew nearly the length of the field and when Greg Janicki allowed the ball to bounce at the top of the United penalty box, it handed Atieno a great chance to test D.C. netminder Milos Kocic. But the tall English striker scuffed his shot wide as the assistant referee raised his flag for offsides.United were enjoying superior possession and Moreno's savvyness at dropping into midfield helped the Black-and-Red weave intricate passing moves on several occasions, with holding midfielder Andrew Jacobson particularly prominent. But their efforts often broke down in the final third and Mella was rarely tested as his back line recovered well.The game erupted into life some five minutes before intermission. First Rhinos captain Kenney Bertz rose to nod a corner kick on target, drawing a sharp reaction save from Kocic -- only for Khumalo to lead United on a quick counterattack that culminated in Fred's cross into the box for McTavish.The United midfielder stretched his leg high to nick the ball into the arm of Ty Harden, prompting referee Abbey Okulaja to point to the spot, to the disbelief and displeasure of the contingent of Rhinos supporters who had made the seven-hour trip south to support their club.Just as he had in D.C.'s league victory against Colorado on Saturday, Moreno calmly dispatched the penalty kick, sending Mella the wrong way to push his team into a 1-0 lead.The Rhinos raced out to a strong start after the break, earning several free kicks around the D.C. box, one of which required Kocic to punch the ball clear in heavy traffic.Energetically pressing forward and getting stuck in wherever possible, Rochester were competing well with their MLS opponents but United's speed posed a constant danger to that approach, as illustrated by another lightning-quick D.C. counterattack that nearly doubled the lead in the 54th minute.Khumalo combined with Ange N'Silu down the left channel before serving up a great cross for Moreno at the far post, and Mella had to produce an excellent leaping parry to keep the score at 1-0.The home fans gave their longest-serving hero a loud ovation in the 58th minute when D.C. coach Tom Soehn -- surely pondering his team's weekend trip to San Jose -- pulled off Moreno and brought on rookie Chris Pontius.A misting rain had settled over the SoccerPlex at halftime and the slick surface might have played a role in the increasing number of fouls and dodgy tackles on both sides in the second half.But D.C. were the guiltier party and in the 68th minute they finally paid the price for conceding so many free kicks in their own end, as a Rhinos set piece from the right flank floated into the United goalmouth and took multiple deflections before Atieno popped up to slot the equalizer past Kocic from close range.Their confidence surging after the goal, the Rhinos continued to test D.C. with direct, aggressive forays forward. But their momentum was interrupted by a gruesome ankle injury to midfielder Darren Kenton -- who was chopped down by D.C. substitute John DiRaimondo -- and the slippery conditions soon influenced the match's course again as United snatched an opportunistic goal after Bertz misjudged a bouncing ball in his own half.The big central defender had played well on the night, but had only himself to blame when the ball bounded off the wet turf and over his head, handing Pontius a clear path to goal. Mella advanced off his line and blocked Pontius' shot, but the rebound fell right into Khumalo's path and the South African stroked a low shot into the unguarded net to restore D.C.'s advantage as the SoccerPlex crowd screamed their approval.Rochester sub Kwame Sarkodie broke in behind Bryan Namoff for a half-decent look at goal in injury time, but he cuffed his shot right at Kocic and United once again rode their luck against minor-league opposition to continue their Open Cup title defense.D.C. will play host to Seattle Sounders FC - who defeated the Houston Dynamo 2-1 in overtime late Tuesday night - in the tournament final at RFK Stadium on September 2.