The Nationals’ Yunel Escobar celebrates after hitting a three-run homer in the first inning Sunday. Escobar had two hits in Washington’s nine-run first — a team record for most runs in a first inning. (Katherine Frey/The Washington Post)

Pittsburgh Pirates starter Charlie Morton entered Sunday’s start against the Washington Nationals on a 14 1/3 -inning scoreless streak, the third-longest active streak of that sort in the majors. In baseball’s best rotation by ERA, he had been the stingiest recently, pitching to a 1.62.

So it was odd when the Nationals knocked him from the game before the first inning ended, batting around and then some in scoring nine runs on eight hits smacked all over the field, including two hit out of it entirely. They went on to win, 9-2, completing their first series sweep in more than a month, against just the second team they have played so far this year that qualified for the 2014 playoffs.

The Nationals have never scored so many runs in a first inning. In fact, the nine runs tied the most this franchise has scored in any inning since moving to the District. They chose an unlikely victim for both the inning-long outburst and the sweep: Morton came to Nationals Park having won five straight starts, and his Pirates entered the series on an eight-game winning streak.

The whole thing started with No. 3 hitter Bryce Harper, as so many things have this season. Seemingly unaffected by the strained left hamstring and deeply bruised quad he is playing through, Harper hit the first pitch Morton threw him into the second deck in right field, beyond the Nationals’ bullpen, for his 24th home run of the year. That scored Yunel Escobar. More on him later.

Next came hits from Clint Robinson, Jose Lobaton and Ian Desmond and an infield single from Michael A. Taylor. His chopper toward third scored Lobaton because the Nationals’ backup catcher had hustled from first to third on Desmond’s bloop single.

Nat's ace Max Scherzer's dominance over the league this season.

“I like the way guys were aggressive offensively early as well,” Nationals Manager Matt Williams said. “Guys are starting to get their legs back a little bit. We’re using the base paths a little more. That’s certainly part of our equation.”

Then came confirmation of the absurdity. Starter Gio Gonzalez fought off an inside fastball and flicked it past Pirates first baseman Corey Hart and up the base line for a double. Gonzalez stood on second, laughing at his luck. He hadn’t had a hit all season.

“I was just tired of [Max Scherzer] trying to be number one hitting,” Gonzalez said.

Next Denard Span walked and Escobar headed back to the plate. He homered to left, a three-run shot for his fourth of the year, and scored his second run in the inning.

The best offensive inning of the season came without the help of four of the eight players Williams expected to have in his regular lineup this season. Jayson Werth and Ryan Zimmerman are on the disabled list. Anthony Rendon and Wilson Ramos had Sunday off. On Thursday, Williams had said “it’s time to step up.”

Lobaton had two hits in two innings. Robinson doubled. Players projected to be on the Nationals’ bench before injuries changed the plan went a combined 16 for 43 (.372) with five RBI this series.

“We have confidence in ourselves,” Robinson said. “The Nationals have done a good job of putting together depth so when you have injuries, you can deal with it and not put yourselves in too much of a rut.”

Jung Ho Kang of the Pittsburgh Pirates strikes out in the fourth inning. Washington didn’t give up a run on Saturday or on Sunday until the ninth inning. (Patrick Smith/Getty Images)

Given room to err, Gonzalez opted not to. A start after he was sent off Tropicana Field without escaping the fourth inning, Gonzalez pitched seven scoreless innings. Because of the lead, he could be aggressive without fear of game-changing consequences. For the most part, he was, scattering four hits amid four strikeouts and two walks.

“I was attacking the strike zone,” Gonzalez said. “I was more aggressive in the zone.”

Matt Thornton completed a scoreless eighth, the 24th straight inning the Nationals did not allow a run — most in team history. That streak ended with two outs in the ninth when Hart hit a two-run homer off Felipe Rivero to give the Pirates their only runs.

“We just played great ball and had some great pitching this weekend. If we can do that, we’d win 162 games,” Harper said. “We’ll go through times where we’ve had ups and downs in this game where the ball just didn’t fall our way. But we had a great weekend. Hopefully we can take this into next week and stay in first place.”

The Pirates came to Nationals Park on Friday night with the best rotation in the majors and with a lineup doing plenty to support it. No one had scored four runs against them since June 9. The Nationals, battered and bumbling when the series began, scored at least four runs in all three games. They finished with 19 runs total, the most the Pirates have allowed in a three-game series this season.

“When you get a team like the Pirates, we know they’re a really good team. We’re facing good starters. You want that challenge,” Lobaton said. “You know now that these guys, everybody here, we can do it.”

Of all the teams the Nationals play this month, Pittsburgh seemed the least forgiving, the least likely to be the fulcrum of a positive pivot. But the Nationals may have used the Pirates as starting blocks all the same. Not much has gone as expected this season.