On the next pitch, Markakis hit a monster home run over the right-field wall for his seventh home run of the season. It was the first home run allowed by Mattheus this year.
"I tried to go in the with the slider a little bit more," Mattheus said. "I tried to get it under his hands, but it ended up hanging down and in. He got the barrel to it. ... I made a bad pitch, he is a good hitter, and that's what good hitters do with bad pitches."
Markakis said he knew the slider was coming his way.
"Slider down and in, I'm pretty sure that's what it was," Markakis said. "I wasn't looking for too much there, I wasn't expecting [the homer]. I was just trying to have a good at-bat, and put a good swing on it. It went out of the park, and we end up winning the game.
The loss dropped the Nationals' record to 23-16.
It was a pitchers' duel between Washington right-hander Edwin Jackson and Orioles righty Jake Arrieta for most of the night.
The Orioles didn't waste time in the first inning against Jackson. With runners on first and third and one out, Adam Jones hit a ground ball up the middle. It ended up hitting Jackson's foot, allowing Xavier Avery to score the first run of the game on the single.
Jackson was almost unhittable after that, however. After the base hit by Jones, Jackson retired 14 of the next 15 hitters he faced.
"I was able to come out and locate pitches when I needed to," Jackson said. "Having a great defense behind me, it always makes the game a little easier. You can come out, attack hitters, and look forward to having great plays made behind you."
Jackson ended up with a no-decision, going eight innings and allowing a run on five hits.
"It was a shame to waste such a great pitching effort by Jackson. He was outstanding," manager Davey Johnson said.
The shame of it was, the Nationals had the leadoff hitter on base six times but had a tough time advancing to second base. It didn't help that they were caught stealing twice, and couldn't get the bunt down or execute hit-and-run plays.
"In nine innings or so, we hit one ball hard," Johnson said. "We had chances, and then our inability to get a bunt down, and bunting at bad pitches. A game like that is frustrating. ... We don't have to try to force the execution of a bunt. But that was a tough one."
It remained 1-0 in favor of Baltimore until the sixth inning, when Ian Desmond hit an 0-1 pitch from Arrieta over the left-field wall for his sixth home run of the season, tying the score at 1.
Arrieta lasted seven innings, allowed the one run on six hits, and struck out nine batters.
Johnson has been a patient man when it comes to the offense, which ranks near the bottom in the Major Leagues. He still believes the offense will get better. It's just a matter of the players not putting a lot of pressure on themselves.
"When you are missing some key guys, it puts more pressure on other guys to be more aggressive and be more run producers," Johnson said. "We need to have patience when we get in those situations. I certainly see the ability. [The Orioles] made good pitches. A lot of times, we got ourselves out by being overly aggressive."