Cincinnati (21-11) swept a three-game set from the Nationals (11-21) at RFK Stadium last month and is 5-1 against NL East opponents this season, but the Reds went 2-3 on their just-completed road trip.
Cincinnati scored 16 runs in victories over Colorado and Arizona, but managed only two combined in its three losses.
“We played about as badly as we could play,” Reds left fielder Adam Dunn said. “Getting two wins is just about a miracle.”
The Reds haven’t scored more than three runs in five of their last eight games.
Cincinnati avoided a third straight defeat by ending the Diamondbacks’ seven-game win streak with a 9-8 victory Sunday.
“I come to the field feeling good one day and terrible the next,” Dunn said. “Maybe this will get me jump started and get the team going again.”
Cincinnati manager Jerry Narron said after Saturday’s 3-1 loss to Arizona that his team would start to hit more, and that was the case in at least the Reds’ next game.
The Reds’ 46 home runs rank third in the majors and their 161 RBIs are the sixth-most.
“We’ve got a good-hitting ball club,” Narron said. “We’re going to start scoring more runs. We just haven’t put it together in the last week or so.”
The Nationals are looking to join the Florida Marlins as the only other NL East team to win a game against Central Division-leading Cincinnati.
Washington is coming off its third series win this season, taking two of three from Pittsburgh after a 1-5 stretch. Rookie Mike O’Connor got his second win in three starts since joining the Nationals rotation, defeating the Pirates 5-4 on Sunday.
Jose Guillen hit a two-run homer during Washington’s five-run third inning to snap a 4-for-25 slump.
“I’m trying to find my swing,” Guillen said. “I need to find some consistency and that’s it.”
Armas is 0-2 with a 6.33 ERA against the Reds, and he’s coming off a poor performance in his last start Wednesday, when he allowed five runs in 2 1-3 innings of a 6-5 loss to Florida. Armas didn’t get a decision.
Claussen is 1-0 with a 1.54 ERA in two career starts against Washington. He allowed seven hits and three runs while striking out five in 6 2-3 innings of a 3-0 loss to the Rockies on Wednesday.
Nationals center fielder Marlon Byrd is hitting .348 in 17 road games this season.