Right fielder Bryce Harper sports a scar above his left eye courtesy of last week’s dugout mishap with a bat. Harper is 0 for 3 with a walk Thursday to drop his average to .227. (Haraz N. Ghanbari/Associated Press)

Perhaps it was a misplaced sense of nostalgia that led the Washington Nationals to conjure up an uninspired loss on the eve of their interleague showdown with the rival Baltimore Orioles. Throughout their brief history in the District, the Nationals have typically either been mired in a losing streak as the twice-yearly Battle of the Beltways rolled around, or about to start one.

Things are surely different this year — in Washington and, for that matter, Baltimore — but if all the evidence you had was the Nationals’ 5-3 loss to the Pittsburgh Pirates in front of 25,757 Thursday night at Nationals Park, you could never hope to appreciate what is at stake this weekend.

No-hit for five innings by Pirates right-hander James McDonald, shaken down for two more homers by certified Nats-killer Andrew McCutchen and witnesses to Jordan Zimmermann’s first non-quality start of the season, the Nationals also suffered the indignity of falling out of first place — thus depriving the Battle of the Beltways, which begins Friday night, of its first-ever matchup of first-place teams.

With a chance for a two-game mini-sweep of the Pirates, the Nationals instead matched a season high with 14 strikeouts (11 at the hands of the unheralded McDonald) and watched as their season-long security blanket — a starting rotation that ranks as the best in baseball by ERA — failed to bail them out.

Handcuffed by McDonald’s deceptive curveball in the game’s first half, the Nationals ignited their Natitude just once all night, in the bottom of the sixth, when they finally chased McDonald with four hits, including Ryan Zimmerman’s two-out, two-run single, followed by Adam LaRoche’s RBI triple, drawing them to within one run at that point. But the Pirates’ bullpen held the Nationals back, giving Pittsburgh a 3-2 edge in the now-completed season series between the teams.

“He had the curve working,” LaRoche said of McDonald (3-2). “When a guy throws 95 [mph], you can’t sit strictly [on] off-speed, so he had that in his back pocket. There’s some deception in his delivery that makes that curveball look really good.”

The Nationals’ hold on first place — a tenuous half-game at the start of the day — perished at the business end of McCutchen’s bat and McDonald’s arm.

McCutchen is one of the best young hitters in baseball, but against the Nationals he is transformed into nothing short of Willie Mays. His two homers off Zimmermann — one into the Nationals’ bullpen in right, the other clear over the Pirates’ bullpen in left — were his ninth and 10th in just 23 career games against the Nationals. And he narrowly missed an 11th in the third inning, when he launched a towering drive to left that stayed just foul.

“We certainly hadn’t figured out how to pitch him,” Manager Davey Johnson said of McCutchen. “He looks awful comfortable in there.”

Far from the efficient, strike-throwing machine Nationals fans have come to know, Zimmermann was sloppy with his four-seamer and needed 107 pitches to slog through his six innings. The most damaging of those pitches was the 0-1 fastball that Pirates catcher Rod Barajas slammed to left for a two-run homer in the fourth.

“That’s the worst I’ve seen him with his command early in the ballgame,” Johnson said of Zimmermann, who fell to 2-4 after a six-inning, seven-hit, four-earned-run outing. “It wasn’t a typical Zim outing. His location was off. He was all over the place.”

“I didn’t feel good at all,” Zimmermann said. “My breaking stuff was good, but when you can’t locate your fastball, you’re going to be in trouble.”

If McCutchen is Willie Mays against the Nationals — he even added a running, juggling, wall-crashing catch of a LaRoche smash in the fifth — McDonald, for five innings, was Cy Young.

Making his first career start against Washington, McDonald struck out seven of the first eight Nationals hitters of the game — including six straight at one point — leading the crowd to cheer sarcastically when Zimmermann, the opposing pitcher, managed to lift a weak flyball to shallow center and thus allow them all to put something other than a K on their scorecards.

“Every now and then,” Ryan Zimmerman said of McDonald, “there’s a guy who gets hot and has a good out-pitch like he did tonight.”

And though the Nationals had ample chances late in the game to reverse the outcome — they stranded six base runners in the game’s final four frames — they remained one big hit shy.

Despite the Nationals’ untimely loss, the Battle of the Beltways has never seen a matchup like the one set for this weekend. The Orioles are 25-14 and holding down first place in the AL East, while the Nationals are 23-15 and now trail the first-place Atlanta Braves by a half-game. Until now, the Nationals and Orioles had never met when both teams were above .500.

Whether or not Orioles vs. Nationals ever becomes a true rivalry, it never had a chance at all until both teams came into it with something tangible to play for — a threshold that finally has been reached.