"We were hoping to get keep it close," Manager Dan Jennings said. "It was good to see our guys come back and tie it up at two. Then Harper's big blow. You make mistakes against good hitting teams, they're going to make you pay. Tonight, we had a hard time keeping the ball in the ballpark. We battled but couldn't climb over the mountain."
Fister hit Derek Dietrich with two outs in the second and Dietrich subsequently scored on an Ichiro Suzuki's 2,900th career hit, an RBI double down the left field line which gave the Marlins a 1-0 lead.
"Certainly great to see Ichiro collect his 2,900th hit," Jennings said. "That's just a tribute to the outstanding Hall of Fame player he is. It's good to see him get the RBI double."
But Washington took its first lead in the fourth, as Bryce Harper opened the frame with a single and scored on an Ian Desmond RBI triple to right field to tie the game at one before a Michael Taylor sacrifice fly gave the Nationals a 2-1 edge.
Justin Bour drew a walk to lead off the bottom of the fourth, and a two-out, RBI single off the bat of Adeiny Hechavarria made it a 2-2 contest.
Yunel Escobar and Jayson Werth both singled with two outs in the fifth before Bryce Harper's 28th home run of the season gave Washington a 5-2 lead.
"With [Harper], it was just a bad sequence against a guy who was seeing the all pretty well against me," Koehler said.
Koehler tossed six innings, allowing six hits and five runs while walking one and striking out four.
"It's the first time in a while Tommy has struggled with command," Jennings said. "Harper is a good hitter. If you make mistakes, he is going to make you pay."
Harper extended Washington's lead to four in the eighth after opening the frame with his second home run of the night, this one coming against Sam Dyson. Dyson also allowed a Michael Taylor solo home run to right field on an 0-2 pitch which made it a 7-2 contest.
Andre Rienzo tossed an inning of scoreless relief but the Marlins were unable to rally late. Miami can still take the series with a victory on Thursday afternoon.
Hero of The Game: Justin Bour (+.061 WPA)
Goat of The Game: Tom Koehler (-.381 WPA)
Play of The Game: Bryce Harper homered in the fifth (+.333 WPA).