Washington had a 6-5 lead entering the bottom of the sixth inning, and manager Manny Acta asked Logan Kensing to protect that lead, but Arizona ended up scoring four runs.
With runners on first and second and no outs, Mark Reynolds hit a shot to shortstop Alex Cintron, who backhanded the ball and tried to get the force at third, but Cintron's throw hit Chris Young's head. The ball rolled near the dugout and allowed Young to tie the game at 6.
One out later, Chris Snyder hit a two-run double to make it an 8-6 game. Ryan Roberts followed with an RBI single.
Kensing has not pitched well since he joined the Nationals on April 29, giving up eight runs in 4 2/3 innings, but Acta had no choice but to use him, as Kip Wells and Joe Beimel were not available. Wells had pitched in three consecutive games, and Beimel threw a lot of pitches in his last two outings. Acta wanted Kensing to pitch as least two frames, but he lasted two-thirds of an inning.
"Logan has good stuff," Acta said, "but he pitches behind in the count. It doesn't matter how hard you throw. Up here the big leagues, when they know a fastball is coming, they are going to hit it. You do have to pitch ahead in order for you to use your breaking ball to make hitters chase. That wasn't the case today for him."
Relievers Mike MacDougal and Jesus Colome are about a week away from being called up. Asked how long the team can stick with Kensing, Acta said, "He doesn't have a deadline here."
The game seesawed throughout. Left-hander Scott Olsen was on the mound when the Diamondbacks took a 1-0 lead, with right-hander Max Scherzer hitting a single to left to drive in Snyder.
The Nationals took the lead off Scherzer in the top of the third, when Adam Dunn and Josh Willingham hit back-to-back home runs.
Before the Nationals could enjoy the lead, Olsen gave up RBI doubles to Eric Byrnes and Snyder in the bottom of the inning to give Arizona a 3-2 lead.
In the top of the fifth, Washington retook the lead when Dunn hit a two-run homer off Scherzer. It was his fourth home run of the series, and he now has 11 for the season. Last year, Ronnie Belliard was the first member of the Nationals to reach double digits in home runs, but that didn't come until Aug. 2.
"Dunn is as good as advertised when it comes down to the offense he brings to us and the respect he brings to the middle of our lineup," Acta said. "He has been an on-base-percentage and OPS machine for us. We enjoy having a guy like him in our lineup."
Before he left the game in the bottom of the inning because of a left ankle injury, Olsen gave up a two-run homer to Byrnes. Olsen ended up pitching 4 1/3 innings, giving up five runs on 10 hits.
Olsen is never shy when it comes to talking about his less-than-stellar outings.
"My outing was awful," Olsen said. "There was nothing positive -- lots of runs, lots of hits. I didn't have great control on my offspeed pitches. It was not very good.
"They fouled off a lot of pitches. It takes a toll eventually. I didn't have my best slider, but they did a good job of taking it. When I threw the fastballs, they didn't miss them."
The Nationals found themselves with the lead once again in the top of the sixth inning. With Esmerling Vasquez on the hill, Wil Nieves scored the tying run on a Nick Johnson groundout. Ryan Zimmerman, who had already extended his hitting streak to 28 games, singled to center to drive in Elijah Dukes.
In the last three innings of the game, the Nationals scored two runs. The biggest blow came when Willingham hit his second homer of the game, another solo shot, in the ninth.
The Nationals are now 7-10 when they score four or more runs in a game, and the bullpen has a lot to do with those losses.
"[We are not discouraged], because they are games that we have lost when we haven't scored," Willingham said. "It works both ways. The bottom line is, if you hit and you don't play defense or pitch, you are not going to win. So you have to do all three to win."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.