SEATTLE (AP) -- It might have been quite a long block for Dustin Ackley, but it appears he has finally turned the corner.
''He was the best college hitter in the (2009) draft,'' Mariners manager Lloyd McClendon said of Ackley. ''Sometimes it just takes time.''
The Mariners had lost three in a row. They also had been 0-11 against the Nationals since the franchise moved from Montreal to Washington for the 2005 season.
Nate Schierholtz also homered for Washington. The NL East-leading Nationals hit 10 home runs in the series, one short of the Safeco Field record for a three-game set established by Cleveland in 2009.
The Nationals have dropped four of six.
The Mariners trailed 3-1 in the fifth when Ackley hit a three-run homer off Tanner Roark (12-9). It was his 11th homer this season.
Ackley also singled home a run, and tied his career high for RBIs in a game. He is hitting .319 with 19 extra-base hits since the All-Star break.
''We're in such a hurry to see guys develop at a rapid pace. I just think he's turned the corner. He believes in what he's doing. He's very confident in the type of player he is and it's showing up on the field,'' McClendon said.
Roark, who lasted 6 1-3 innings and gave up four runs and 11 hits, said it came down to ''just one bad pitch'' to Ackley.
''I made a mistake and threw a fastball in,'' Roark said. ''It was first and third and one out, threw a fastball, just cut it a little bit and it was in his sweet spot.''
Hisashi Iwakuma (13-6) went six innings, allowing three runs and five hits. He struck out six and walked none.
''I didn't have a good feel for the ball today,'' Iwakuma said through his translator. ''I kind of struggled. But I was able to change speeds and move the ball around.''
Over his last 11 starts, he is 8-2 with a 2.35 ERA and has 72 strikeouts and five walks.
Relievers Charlie Furbush, Danny Farquhar and Yoervis Medina combined to work two hitless innings. Fernando Rodney gave up a leadoff single to Harper in the ninth before getting his 39th save in 42 chances.
Brad Miller and Austin Jackson each had three hits for the Mariners.
Harper hit a 418-foot, solo homer in the second, the first run Iwakuma had allowed against National League teams in 23 innings.
Harper hit a 416-foot shot to straightaway center in the fourth for his 10th homer this season. It was his fifth career multiple home run game.
''I don't think he's done anything different I just think he's getting stronger,'' Nationals manager Matt Williams said of Harper, who missed 57 games with an injured thumb.
''His hand is getting stronger. It allows him to stay back and calm and keep his body down a little bit. We've seen that over the last couple weeks. It's getting there,'' he said.
Harper was 6 for 12 with three home runs in the series.
The Mariners added a run in the eighth on Endy Chavez's RBI double.
McClendon, who missed the previous two games to attend his daughter's wedding in Indiana, returned just before game time and assumed his duties. Bench coach Trent Jewett had been serving as interim skipper.
''It was very excruciating, to say the least,'' McClendon said. ''My heart was with these guys but my heart is also with my daughter as well.''
Nationals: Washington, which has the best record in the NL, open a three-game series Monday night at Dodger Stadium, where Los Angeles has the second-best record in the league. Nationals LHP Gio Gonzalez (6-9) faces RH Roberto Hernandez (8-9).
Mariners: Seattle opens a three-game series at Oakland beginning Monday. Chris Young (12-6) will be opposed by Jason Hammel (1-5). It's the first of 18 road games for the Mariners in September against only nine home games.
The Mariners had their annual Salute to Kids Day in which youngsters run onto the field before the game, greet the players coming on the field, get an autographed baseball then dash off. But one enterprising youngster, Jason Clark of Seattle, not only got his ball signed by Robinson Cano, he marched him over to the second-base bag and took a selfie with him, to the delight of the fans.
THE BIG NUMBER
13 - Walks issued by Iwakuma this season to go with his 13 wins in 23 starts. Since 1893, there have been three pitchers to qualify for the ERA title that finished the season with more wins than walks: Christy Mathewson (1913-14), Slim Sallee (1919) and Bret Saberhagen (1994).