Dee Gordon singled to start the Miami Marlin's first, stole second, took third on a balk and scored on an RBI single to left by Christian Yelich, who shot a 1-1 slider through Washington's drawn-in infield.
A walk to Giancarlo Stanton put two runners on, but Nationals' starter Tanner Roark retired Justin Bour on a grounder to first that moved both Yelich and Stanton into scoring position.
Marcell Ozuna stepped in with two on and two out and lined a 2-2 slider to left-center for a two-run single that made it 3-0 Marlins after one.
Miami added a run in the second on an RBI groundout and the fifth run they scored with Tanner Roark on the mound (4 ER) came on a solo home run by Stanton in the fifth, when the Marlins' slugger took a 90 mph 2-0 heater for a ride to center field. 5-0.
Dusty Baker told reporters after the 6-1 loss that he thought Roark's stuff looked okay, but a few mistakes cost him.
"What hurt him was that two-out RBI by Osuna," Baker said, "and then we brought the infield in, because you can't just give away runs when you're facing [Jose] Fernandez. A lesser pitcher, we'd have probably conceded a run.
"The ball [Christian] Yelich would have been a hit anyway. He just hit it in a perfect spot. And, boy, it's hard to keep 'Big Boy' in the ballpark too, over there, I mean, forever, you just can't do it. We'll come back tomorrow and we'll be much better tomorrow."
"I mean, one bad pitch to Ozuna, that's about it," Roark explained when asked what went wrong in the outing in Marlins Park.
"Everything else felt good. And then obviously the 2-0 fastball, tried to go in on Stanton and I didn't get it there and when you make mistakes to guys like that they hit the ball far. That's what happened."
Through two starts against the Nationals' NL East rivals thus far, Roark has allowed 16 hits and nine runs, seven earned in 10 innings.
"They're a good-hitting ballclub from top-to-bottom," Roark said, "so you've just got to execute your pitches and keep them uncomfortable up there at the plate, that's the main thing."
The loss was the Nationals' second straight after a seven-game winning streak. They try to get back to their winning ways tonight.