Ian Desmond drills a home run off of the Phillies’ Roy Halladay — his team- leading eighth of the season — and second in as many days in Philadelphia. (Hunter Martin/Getty Images)

The final, fraying vestiges of the Philadelphia Phillies’ superiority over the Washington Nationals unraveled Tuesday night at Citizens Bank Park. The Phillies sent to the mound Roy Halladay, more obelisk than pitcher, their ace, the right-hander whom the Nationals had never beaten. He proved just as helpless as the rest of his teammates from preventing the sudden reversal of a rivalry.

The Nationals knocked around Halladay in a 5-2 victory even after third baseman Ryan Zimmerman was made a late scratch because of a precautionary manager’s decision. Halladay chucked a rosin bag after he allowed a home run. Bryce Harper, using three years of study and a spring training planning session, drilled a two-run triple off him. The Philles’ catcher was ejected. Halladay’s world, like the recent matchup between Washington and Philadelphia, had been turned upside down.

The Phillies built their five-title reign over the National League East on the Nationals’ backs, but those days ended abruptly. The Nationals have beaten the Phillies in nine of their past 10 games, including four out of five this season and six straight in Philadelphia. Washington leapfrogged the Atlanta Braves into first place — six games clear of the last-place Phillies, the only team in the division with a losing record.

“The Phillies, as far as I’m concerned, are still the king of the mountain,” Nationals Manager Davey Johnson said. “Nobody’s really knocked them off that mountain. My guys know if we want to play with the best, we’ve got to beat these guys. And we’ve been doing a pretty good job.”

The Nationals (26-17) won behind one run over six innings from Jordan Zimmermann, who saved a depleted bullpen by throwing the final nine of his 107 pitches in a 1-2-3 sixth inning. He also added two hits as he earned his first win in five starts against the Phillies. It came against Halladay, who before Tuesday night had gone 9-0 with a 2.13 ERA against the Nationals.

The Nationals smacked nine hits off Halladay in six innings, including home runs by Ian Desmond — who drilled his team-leading eighth — and Rick Ankiel. They attacked him early in the count, not allowing him to dice them with his arsenal of sinkers, cutters, sliders and change-ups, all of which look the same until they dart a few feet in front of the plate. Eight of their nine hits came within the first three pitches of an at-bat, four on the very first pitch.

“Anytime you’re facing a Hall of Fame-caliber pitcher that doesn’t make many mistakes when he pitches, you’ve got to be aggressive,” Johnson said. “He’s going to be hitting the corners and use the whole strike zone. You better be up there swinging.”

In the first three innings, Halladay allowed only two soft singles and struck out two as the Phillies (21-23) took the lead on an RBI ground-rule double by Hector Luna. The Nationals had wrested control of the rivalry, but Halladay would surely take it back.

The third inning stunned the 45,569 at Citizens Bank Park into near silence. Zimmermann ripped the first pitch into left for a single. Steve Lombardozzi lined another hit, bringing Harper to the plate.

In spring training, hitting coach Rick Eckstein had asked Harper — a 19-year-old who thinks like a veteran in the box — what he would do if he faced Halladay in a game this season. Harper said he would expect to see a first-pitch curveball, Halladay simply trying to get it over for strike one. Eckstein nodded and said, “I think you’re right.”

“I’ve been watching him for about three years,” Harper said Tuesday night. “He throws first-pitch curveballs to so many people, and they just let it get over the plate. I was just really trying to get something up in that situation and get something going.”

Halladay fed him the curve, 76 mph. Harper did not miss his chance. He lambasted a line drive into the right-center field gap. As Zimmermann and Lombardozzi scored, Harper sprinted around the bases. He did not draw a throw, but slid headfirst into third anyway. The Nationals led, 2-1, and Harper had his third triple.

“He hung a curveball to the kid, and the kid smoked it,” Johnson said. “That was really the turning point, as far as I’m concerned.”

LaRoche drilled a sacrifice fly to center, bringing up Desmond, who had already hit three home runs in the past week. Halladay fell behind, 2-0, and threw a curveball trying to get back into the count, like the one he threw Harper. Desmond hammered a line drive deep into the left field seats. The Nationals led, 4-1.

Ankiel added to Halladay’s torment. He walked to the plate to lead off the fourth six for his last 46 with 19 strikeouts. He unloaded on a first-pitch cutter, sending it over the center field fence. As if things could get worse for Halladay, the home plate umpire ejected catcher Carlos Ruiz as he argued a ball call for Danny Espinosa.

“I’ve seen him sharper,” Ankiel said. “But he’s still is who he is. It seems like in the past, consistently he’s been sharper. Tonight, he might have made more mistakes than normal. He’s human.”

The Nationals had given the habitually under-supported Zimmermann more than he needed. Zimmermann squirmed out of only one serious jam. In the third, right after the Nationals’ decisive onslaught, the Phillies put men on second and third with one out. Zimmermann struck out Hunter Pence and induced a lazy fly to center from backup catcher Brian Schneider to end the inning.

The only intrigue by the end of the night came when the bullpen gates opened in the ninth inning. Johnson gave the first post-Henry Rodriguez save opportunity to Tyler Clippard. He struck out Peter Orr and induced groundouts by Juan Pierre and Placido Polanco to earn his second career save.

“It feels good getting that final out and just hearing crickets out there,” Clippard said.

The crowd had actually thrown boos at the home team. Desmond would later provide a reality check. “It’s May, you know?” he said. “We’ll see what happens.” Still, the night felt different. As the teams retreated to their clubhouse and fans filtered out of the stadium, the public address speakers blared “Yesterday’s Gone.”