Bryce Harper rounds the bases after his fourth-inning home run. The Nationals took two of three from the playoff-hopeful Mariners and now head to Los Angeles to face the Dodgers. (Otto Greule Jr/Getty Images)

The Washington Nationals’ weekend romp through Seattle, a home run binge unprecedented in the team’s brief history, continued without a snag at the start of Sunday afternoon. They sent three more homers into the Safeco Field seats. Nate Schierholtz, the ninth man in the lineup, clubbed one over the center field fence for his first hit as a National. Bryce Harper blasted a baseball off a restaurant.

The power surge veered off course, though, and a potential sweep turned into a 5-3 loss to the Seattle Mariners. The Nationals could not overcome the go-ahead, three-run homer Dustin Ackley drilled off right-hander Tanner Roark in the fifth inning. The Nationals couldn’t close the gap in three innings against the Mariners’ hellacious bullpen, which leads the majors in ERA.

“Just one bad pitch,” Roark said. “That’s all I can say.”

The loss aside, Harper’s performance may have been the most significant development of the afternoon. Harper went 3 for 4 with two home runs and a single to lead off the ninth inning, continuing a personal hot streak that bodes well for the Nationals entering September.

Since Aug. 8, the day after he blasted a walk-off homer at Nationals Park, Harper has gone 24 for 78 with six home runs and six walks. As Harper moves further away from the left thumb surgery that caused him to miss 57 games, he has grown stronger and regained his timing.

Nationals starter Tanner Roark falls to 12-9 after yielding four runs and 11 hits in 6 1/3 innings. (Otto Greule Jr/Getty Images)

“It’s significant that he feels good,” Manager Matt Williams said. “It’s the case with everybody — you miss that kind of time, it takes you time to get your rhythm and timing and strength back. You see a little bit better bat speed with not as much effort.”

Sunday, the Nationals couldn’t turn Harper’s performance into a victory. They still claimed the series, a recovery from the sweep they endured in Philadelphia. After they set a team record with 10 home runs in a three-game series, they headed to Los Angeles for three games against the first-place Dodgers with a six-game lead in the National League East over the Atlanta Braves, who won Sunday evening.

Roark allowed 11 hits in 61 / 3 innings, but most were flares that fell in a vast outfield, and only one caused significant damage. Roark took the mound in the fifth with a 3-1 advantage. With one out, Austin Jackson blooped a single to center field, which put two runners on base for Ackley.

Roark fell behind Ackley, 2-0. Catcher Wilson Ramos called for a fastball and set up his mitt over the outside corner. Roark pulled the pitch across his body, firing it low and inside, where left-handed batters most often crush pitches.

“I didn’t stay through it,” Roark said. “Just yanked it.”

Ramos reached his left arm all the way across the plate but he never had a chance to catch it. Ackley blasted it five rows over the right field fence, a bolt that turned a two-run lead into a 4-3 deficit.

“It’s the only really bad pitch he made,” Williams said. “It was just the one pitch. Today, I think he was sharper than last time out. It’s a question of one pitch sometimes.”

With just over a month left in the regular season, the Nationals have a solid starting rotation with the best ERA in the National League. Assuming the playoffs started tomorrow, the Post Sports Live crew debates which Nationals pitcher should start Game 1. (Post Sports Live/The Washington Post)

The game had begun like the others this weekend, with the Nationals finding new places in cavernous Safeco Field to deposit baseballs.

Harper faced right-hander Hisashi Iwakuma, one of the most underrated pitchers in baseball, for the first time in the second inning. Iwakuma hung an 0-1 curveball at the top of the strike zone. Harper stayed back, keeping his weight on his back foot, and annihilated the pitch. The crowd gasped. The baseball tested the integrity of the glass windows around the Hit It Here Cafe in the right field upper deck.

“It’s been impressive,” Schierholtz said. “You can’t argue with that, especially this series. It’s nice to see him getting hot right now.”

As Harper stepped to the batter’s box for his second at-bat, a heckler near the Nationals’ dugout who had been riding him all game shouted, “No more homers for you!” Iwakuma threw him three consecutive balls to begin the at-bat. Iwakuma had walked only 13 hitters, less than one per nine innings, all season, so Harper knew he could expect a strike.

Iwakuma threw Harper a 3-1, thigh-high splitter over the outer half of the plate. Harper mashed a line drive that disappeared over the center field fence. For the fifth time in his career, Harper had drilled two home runs in one game. His 10th homer of the season gave the Nationals a 3-1 lead.

“I feel pretty good up there,” Harper said. “I’m just trying to put together some good ABs and get my work in every single day. Come in, stick with the same plan, the same routine.”

“I just think he’s getting stronger,” Williams said. “His hand is getting stronger, which allows him to stay back and calm his body down a little bit. We’ve seen that over the last couple of weeks. It’s getting there.”

Once the Nationals fell behind, the threat of Harper altered Mariners Manager Lloyd McClendon’s strategy. Iwakuma had thrown only 86 pitches through six innings. But with Harper leading off the seventh, McClendon pulled one of his best starters to make Harper face a lefty. Charlie Furbush trotted in from the bullpen and struck out Harper with a 92-mph fastball after throwing him six straight fastballs.

Furbush had done his job, but the effect of chasing Iwakuma would be felt. Danny Farquhar replaced Furbush. Farquhar struck out Ramos, but strike three bounced in the dirt and scooted to the backstop, allowing Ramos to reach. With one out, Asdrubal Cabrera walked to put two runners on base.

But the Nationals could not take advantage, even with two left-handed hitters. Schierholtz and Denard Span both tapped grounders to the right side, stranding two runners. The Nationals would not create a better chance to inch back into the game. Having finished August at 19-10, the Nationals chose to look forward rather than dwell on a rare loss.

“Take it down to L.A.,” Harper said. “And see what we can do down there.”