PITTSBURGH — Bryce Harper has known for about a year that PNC Park was the only stadium in the National League where the Washington Nationals star hadn’t gone deep.

One vicious swing on Tuesday night checked it off the list.

Harper sent a laser to the last row of seats in right field in the top of the ninth for his 13th home run to give Washington’s erratic bullpen a little extra breathing room in an 8-4 win over the Pittsburgh Pirates. Only a protective fence prevented the ball from ending up in the Allegheny River.

Not that Harper was aiming or anything.

“I don’t worry about hitting the ball in the water,” Harper said. “(Former manager) Matt (Williams) used to say ‘It’s not how far, it’s how many.’ Just trying to put the ball in play and sometimes it goes over the fence.”

Or a lot of the time if you happen to be Harper or the other bats in baseball’s most explosive lineup. Such are the riches for the Nationals at the moment that Ryan Zimmerman went 2 for 5 with a two-run double ... and was leapfrogged by Harper for the top batting average in the majors (.388). Zimmerman’s two RBI pushed his total to a major league-leading 38, two clear of Harper.

Jayson Werth added three hits and is batting .457 (16 for 35) in his last nine games. Wilmer Difo lined a shot to the seats in center field for his first home run of the season for Washington, which has won four of five.

Stephen Strasburg (4-1) allowed three runs in six-plus innings, walking one and striking out three on a night when his stuff was more ordinary than electric.

“I got a little bit better command wise, but I felt like I was grinding through it all night,” Strasburg said. “It wasn’t my best, but they picked me up.”

Josh Harrison had two hits for the Pirates, including his sixth home run of the season, and also avoided a tag at second base with an acrobatic slide in the sixth. Chad Kuhl (1-4) remained winless since the first week of the season.

The Nationals are in the process of running away with the NL East not even a quarter of the way into the season thanks to the best offense in baseball. The lineup wasted little time getting to Kuhl, who hasn’t won in six weeks.

Trea Turner led off with a double and scored on Werth’s RBI single. Strasburg led off the third with a single, moved to second on a single by Werth, and Zimmerman followed one batter later with a double that put the Nationals in front 4-1. Michael Taylor tripled with one out in the fourth, and Difo, who came in batting just .196, sent Kuhl’s pitch to the seats for his first of the season to make it 6-1. Kuhl’s ERA ballooned to 6.69 in eight starts.

“I hate losing,” Kuhl said. “I hate falling behind hitters.”

Washington’s bullpen began the day with a 5.53 ERA, third worst in the majors. It flirted with letting a lead slip away in the seventh when the Pirates scratched across three runs thanks in part to a throwing error by Zimmerman. Blake Treinen — who began the season as the closer before being demoted — got Harrison to hit into a double play, then struck out Andrew McCutchen to end the threat.

Enny Romero pitched a perfect eighth and Koda Glover worked around two baserunners in the ninth.