Marquis allowed one run on six hits in six strong innings to notch his second consecutive victory.
"He pitched out of trouble a few times today," manager Jim Riggleman said. "That's what he's got to do when he gets in trouble, get some ground balls, get guys off the barrel of the bat. He did that at the right times today. He's been good, he's had several outings now where this is the guy we thought we were going to get."
Marquis made his sixth start after a three-and-a-half month stay on the disabled list after having bone chips removed from his right elbow. With the win, Marquis upped his record to 2-7. It was also Marquis' 11th career win against Pittsburgh -- his most against any big league team.
"Start-by-start I feel like I'm getting to where I need to be," Marquis said. "Obviously early on I was hurting, and since the surgery I feel like Jason Marquis more day-by-day. The last four starts have been right where I want to be, although I'd like to go a little deeper into games, but I've just got to minimize my pitch count and that will happen."
Marquis outlined why he was successful Sunday afternoon.
"Getting ahead early [with] strike one really was important," Marquis explained. "I felt like I've been getting my sink back day-by-day, slot-by-slot -- so you get a little more confidence in it and you're able to pound the zone early and often."
Pirates starter Charlie Morton endured another rough outing. He was tagged for six runs (two earned) on eight hits in 3 2/3 innings. He took the loss falling to 1-11 on the season.
"The inning Morton gave up the runs, he couldn't really get a ball hit at somebody," Pirates manager John Russell said. "They kept finding holes. But for the most part, I thought he was better. Still not where we would like to get him, but it was a step in the right direction."
Washington manufactured an unearned run in the top of the first. Nyjer Morgan singled and advanced to second base on an errant pickoff throw by Morton. Morgan was sacrificed to third and scored on a groundout by Ryan Zimmerman.
After resting a sore hamstring on Saturday night, Dunn made his presence felt in the Nats' lineup. He smacked an opposite-field home run -- his 34th of the season -- to lead off the fourth.
"I've been feeling good for about a couple of weeks now, and it just one of those little streaks that I go into where I'm seeing the ball pretty good," Dunn said.
The Nats tallied four more runs in the inning after back-to-back singles from Roger Bernadina and Michael Morse, Adam Kennedy reached base on an error by first baseman Garrett Jones and Bernadina scored. Ian Desmond then drove in Kennedy. Zimmerman singled up the middle, driving in Wilson Ramos and Marquis and chasing Morton. When the smoke cleared, Washington had a 6-0 lead. Bucs right-hander Daniel McCutchen ended the uprising in which the Nats sent 10 men to the plate in an inning for the second game in a row.
Pirates rookie third baseman Pedro Alvarez led off the bottom of the fourth with his 11th home run, cutting the Nats' lead to 6-1. But Marquis would keep the Pirates off the scoreboard for the rest of his outing.
"The key today was Jason," Dunn said. "Throwing strikes and getting strike one, and he was able to work quick and keep the defense in the game."
After a rocky start to his tenure Marquis appears to be heading toward solidifying his role as a veteran presence in the rotation.
"It was tough for him early," Riggleman said. "I know he was not satisfied. Everyone was concerned about his results, but once he got his arm straightened out and has been able to go out there on a regular routine and throw his pitches, he's been much better."
Washington added to its lead in the top of the sixth against Pirates reliever Joe Martinez. Morgan drew a walk, Desmond reached on an error by Martinez. Morgan scored on a single by Zimmerman, his fourth RBI of the contest. Dunn singled to knock in Desmond.
Nats reliever Tyler Clippard pitched two scoreless frames and righty Drew Storen hurled a scoreless ninth inning to close out the game for Washington.