With Miami's win, the Braves clinched the National League East. The Nationals were also dealt a setback in their Wild Card bid.
"We talked about that earlier when we got here, that we were going to play teams down the stretch that are trying to get in the playoffs or are in the playoffs," Koehler said. "This is a good experience for us. Obviously, we're not in it. But to get to play against teams who are playing for something that means something, it's good -- because we're going against guys who are really locked in."
Washington was a perfect 8-0 at home against the Marlins this season until the Marlins claimed the first game of the twin bill.
The doubleheader was scheduled after Saturday night was washed out, after both sides waited out a 3-hour and 50-minute rain delay.
"It's nice to get a win here, the first one of the year," manager Mike Redmond said. "I was happy more with [how we responded] after the long night last night -- we came in ready to play. We had the big hit by Stanton to get a couple of runs. It was nice for us to go out there and battle and pick up just enough runs."
At 57-98, the Marlins have matched their second-highest loss total in franchise history. The 1999 squad also dropped 98, and the franchise high is 108 in 1998.
The Marlins are playing for pride, and looking to leave their mark on the playoff chases. The Nationals entered Sunday one loss away from being mathematically eliminated in the NL East. They are also scrambling to remain in the Wild Card picture, which now becomes even more daunting.
Miami closer Steve Cishek worked a perfect ninth inning for his 32nd save. The right-hander matched a club record of logging 27 saves in a row, which was initially established by Todd Jones in 2005.
The crowd of 35,101 was frantically rooting on the Nationals in the ninth inning, but Cishek was able to retire Jayson Werth on a soft pop fly to first base for the final out.
"It was crazy out there," Cishek said. "I had a little more adrenaline pumping. It felt like a playoff atmosphere. The fans were really into it. Those guys are really digging in there, and every at-bat was huge for them. It felt good to come out and win a game like that. The atmosphere was just awesome to be a part of a game like that."
The Marlins may be last in the Majors in home runs, but they used the long ball to grab control early off Dan Haren. Stanton blistered a two-run homer in the first inning, and Yelich connected on a solo shot in the third.
For the second straight start, Koehler was in command. The right-hander was coming off an eight-scoreless-inning performance against the Mets. He had his string of 15 straight scoreless innings snapped in the sixth inning on Ryan Zimmerman's two-run homer.
"They're a good team," Koehler said. "You could see -- you walk a guy, and it turns into a two runs real quickly. You really had to stay focused throughout the entire game and just try to execute the game plan."
Koehler is set to make one more start, which is scheduled for Friday at home against the Tigers. But a few days ago, it was questionable if the right-hander would be ready for Sunday. Last Monday, while stacking a dumbbell in the weight room in Philadelphia, his left index finger was gashed, resulting in two stitches.
Saturday's rainout may have worked to his benefit, because his finger on his non-throwing hand felt better with the extra day of rest.
In six innings, Haren was charged with three runs on eight hits.
In the seventh inning, Juan Pierre lined a pinch-hit single to left off Ryan Mattheus. It was career hit No. 2,216 for Pierre, moving him past Joe DiMaggio for 174th all-time.
Pierre then used his speed to help Miami manufacture a run. He swiped second and moved to third on a wild pitch. After Chris Coghlan walked, Donovan Solano lined a sacrifice fly to left. The speedy Pierre scored standing up ahead of Bryce Harper's throw, giving the Marlins a 4-2 advantage.
Silenced in a two-hit shutout at the hands of Jordan Zimmermann in the series opener on Friday night, the Marlins waited out Saturday's rain delay before the game was postponed.
The Marlins didn't have to wait long to redeem themselves, as they matched their hits total for Friday in the first inning, while also pushing across a couple of runs.
Yelich delivered a two-out double down the right field line, and Stanton smashed a two-run homer to right-center. It was the slugger's 24th shot of the season.
Yelich padded the advantage to three runs with a two-out home run in the third.
The Nationals closed the gap to one run in the sixth inning on Zimmerman's two-out, two-run blast off Koehler, after Denard Span kept the inning alive by drawing a walk.
Zimmerman continues to hurt Marlins pitching. The home run was the 23rd in his career against Miami, his high against any opponent. His 76 RBIs match a personal high, which also equals his tally against the Braves.
"[Koehler] was throwing strikes," Zimmerman said. "He had a good sinker today and, obviously, a low pitch count. He started to miss a little bit toward the end, but before that, he just really didn't miss over the plate at all."