Washington arrived in Cincinnati on a four-game win streak but dropped the series opener with the Reds, who scored four runs on Nationals' lefty Gio Gonzalez in a long bottom of the fourth.
Reds' hitters singled the southpaw to death and added another run on a solo home run in the fifth before the Nats got on the board.
Dusty Baker tried to jumpstart the Nationals' offense late, after Jayson Werth hit a solo blast to make it 5-1, bringing Wilson Ramos and Clint Robinson off the bench, but Zack Cozart hit a solo blast, 6-1, and after Ramos doubled and scored, 6-2, Adam Duvall hit another homer to put the Reds ahead, 7-2.
"I was looking for some offense, big time," Baker said when asked about the late-game changes, which included using his backup catcher.
"When you're playing from behind, then you have to do things that you might not ordinarily do at that stage and time of the game and so at that point I think the score was still 5-2 and we had action on coming back especially with this big- inning team that we have and then they matched us a run and you can't trade runs with them late in the game like that.
"We were just trying to put some offensive guys up there that, especially in this ballpark, that can hit the ball out of the ballpark."
Anthony Rendon put the Nationals up early with a two-run home run in the second on Saturday, but Jay Bruce cut the lead in half with a solo home run off Stephen Strasburg in the home-half of the inning.
Ben Revere and Bryce Harper walked in the third and fourth innings, respectively, but both were thrown out by Reds' catcher Tucker Banhart when Baker put them in motion trying to make things happen.
Joey Votto hit a two-run blast in the sixth to put the Reds up, 3-2, and after Danny Espinosa tied things up at 3-3 with a solo home run, Duvall hit a three-run homer to put Cincinnati up for good, 6-3.
Baker talked after the second straight loss about running into outs when they had runners on.
"The thing about it is, we're not scoring a whole bunch of runs," Baker said. "You can wait around for the homer, if you want to, it doesn't work that way sometimes.
"When you steal a base, everybody is happy, when you get caught they say, 'What happened?' Or, 'Why don't you try to have some movement?'
"That young man, Tucker Barnhart, I had him and he can throw the lights out of the ball, but it was a situation where it is called 'stealing'.
"It's not called 'giving' or 'taking', it's called 'stealing', and if you steal sometimes, well, even the best of thieves they get caught sometimes, just don't lose sight of the fact that it is called 'stealing'."
In Sunday's series finale, the Nationals fell behind early, 5-0, but rallied to tie it and eventually take the lead with Daniel Murphy and Ramos homering and Stephen Drew and Ben Revere singling in runs in a five-run fourth.
A two-run error and sac fly put the Nats ahead, 8-5, after five, and Murphy and Ramos drove in runs with singles to make it 10-5.
The Reds rallied late, however, with Bruce hitting a three-run home run in the seventh and Brandon Phillips hitting an RBI double in the ninth to get Cincinnati within a run in what ended up a 10-9 Nationals' win.
"Honestly, I was thinking about Fred McMurray, and 'My Three Sons'," Baker joked after the game.
"But I've got four sons over there that wreaked havoc on us this series in Brandon and Joey and Jay Bruce and Cozart, and man, I was like, 'Did I teach you guys to do that?"
He also talked about his new charges waking up offensively with Revere going 2 for 5, and Michael A. Taylor lining a double to the wall in left while his middle of the order bats (Harper 3 for 5; Murphy 3 for 5 and Ramos 3 for 5) came up big in the sweep-avoiding win.
"That was big for Harp," Baker said. "That's the first time he's had three hits in a long time. And you like to see Ramos and Murph -- he was cold the first couple games here. He came back to life. And it was big for Ben Revere to get a couple hits and Michael Taylor hit a double, so hopefully this is big for their confidence and it will help us, big time."
Will the Nationals offense keep rolling tonight in the series opener with the White Sox in Chicago? The first game of the three-game set on the south side starts at 8:10 PM EDT.