WASHINGTON -- Tommy Kahnle said his changeup was working for him as he pitched the ninth inning Friday. But what he throws is usually not his call.

"I just go with what (Nick) Hundley puts down, I guess," said Kahnle, with a grin.

And the change is what Hundley called for and the right-hander used to strike out Washington slugger Bryce Harper for the final out with a runner on first as the Colorado Rockies beat the slumping Nationals 5-4.

It was the first save in the big league career of Kahnle, who has pitched 83 games out of the bullpen the last two years. And Friday was the 26th birthday for the Latham, N.Y., native.

"I am excited about it," said Kahnle, a product of Division II Lynn University. "But more importantly, we got the win. The changeup was really working well tonight."

Said Colorado manager Walt Weiss: "That was a big-time save. He threw a nasty changeup that last pitch (to Harper). We fight, man."

The other hero for the Rockies was Carlos Gonzalez, who hit a grand slam in the eighth off former Washington closer Drew Storen to give Colorado the 5-4 lead. It was the 23rd homer of the year for the lefty and the first grand slam of the season for the Rockies.

"He has been great all year," Gonzalez said of Storen, who had retired 19 batters in a row before allowing a baserunner Thursday. "It was just a rough outing for him. That was a big moment."

Gonzalez had two hits and is batting .282. "He is one of those elite players," Weiss said.

Storen had 29 saves for the Nationals this year before the Nationals traded for Jonathan Papelbon, who is now the closer.

"He is a great competitor for us. I am not worried about him at all," second baseman Danny Espinosa said of Storen.

Said Weiss of Storen: "He is lights out (most of the time)."

Washington manager Matt Williams said Storen has had a great year. "Tonight just wasn't his night. He made a mistake to a guy that's recently been really hot and has power."

More of a concern for the Nationals is that they have lost six of their last eight games and are 2-3 on a current homestand against sub .500 teams in Arizona and Colorado. But right fielder Harper doesn't seem concerned as the Nationals have three everyday players, left fielder Jayson Werth, first baseman Ryan Zimmerman and infielder Anthony Rendon, back in the lineup after missing a lot of time on the disabled list.

"We have a great lineup," Harper said.

The winning pitcher was Scott Oberg (3-2), who pitched a scoreless seventh.

Colorado (46-61) won for just the fourth time in 11 games while the Nationals (56-52) continue to struggle since the All-Star break.

Washington starter Jordan Zimmermann, after giving up 11 runs in his last three starts, allowed one run in 6 2/3 innings but did not figure in the decision.

Right fielder Harper had an RBI groundout in the first, singled and scored in the fourth and walked and scored in the sixth. He also walked in the seventh but was stranded as the Nationals left the bases loaded as Zimmerman struck out to end the frame.

The Nationals assumed a 4-0 lead in the sixth when Werth drove in a run with a sacrifice fly and shortstop Ian Desmond had an RBI double to left to score Harper.

Colorado trimmed the margin to 4-1 in the seventh on an RBI single by first baseman Ben Paulsen. His hit scored third baseman Nolan Arenado, who had doubled. Arenado's third hit in the eighth was a slow roller down the third-base line, and then Gonzalez followed with his grand slam to overcome a 4-1 deficit.

Colorado starter Jorge De La Rosa gave up four runs (three earned) and before Oberg took over in the seventh.

Espinosa had two hits and scored twice. Arenado also made two nice plays to retire speedy Washington center fielder Michael A. Taylor on slow rollers.

Harper drove in the first run with a groundout in the opening inning after Espinosa doubled and then took third on an error when the ball got past left fielder Kyle Parker in the corner.

The Nationals made it 2-0 in the fourth when Harper singled with no outs and scored when Zimmerman lined a double up the alley in right center field.

NOTES: Rockies RHP Eddie Butler (3-8, 5.45 ERA), a Virginia native, will start Saturday against Nationals RHP Stephen Strasburg (5-5, 5.16), who is slated to come off the disabled list. Strasburg went on the DL July 5 with a left oblique strain. ... Since June 1, Rockies OF Carlos Gonzalez (18) and 3B Nolan Arenado (14) rank first in the National League in homers in games through Friday. ... Washington RF Bryce Harper, who was on base after all five at-bats Thursday with three walks and two singles, has reached in 32 of his last 33 games going into Friday. He was leading the majors in slugging at .666 and the National League in homers with 29. ... Rockies assistant to the general manager Vinny Castilla hit the Nationals' first homer in Washington on April 14, 2005, against Arizona.