Ignited by Petersen's single to left, the Marlins pushed two across in the eighth inning, and held on to snap a three-game losing streak.
"It's good experience," Petersen said. "I'm glad the team won. I got that hit. It was just an awesome experience."
Promoted from Triple-A New Orleans, Petersen was brought in to help energize the Marlins, either starting or coming off the bench. The left-handed-hitting outfielder figured he would be better off following one of the top bench players in the game. So he shadowed teammate Wes Helms, the franchise's all-time pinch-hit leader.
"He asked me my routine," said Helms, called Uncle Wes by some of the younger players. "He goes, 'Hey, do you mind if I follow you around today?' I said, 'No, I want you to.'
"He came in and stretched when I stretched. We threw each other flips in the cages. ... I said, 'I'll let you know if you've got to get ready earlier.' He did all of this with me. He said when he is on the bench, he wants to follow me and do what I do. He was ready. He was loose."
After being swept in three games at home by the Giants, the Marlins were looking for a bounce-back effort in the start of their six-game road trip.
The Marlins were able to make the most out of seven hits and provide enough cushion for Chris Volstad (3-2) to beat the Nationals for the second time in five days. The 23-year-old right-hander was solid, giving up two runs on four hits in seven innings.
Jorge Cantu provided a home run and sacrifice fly, on a night he broke out of an 0-for-19 slide.
"I thought Volstad was the player of the game," manager Fredi Gonzalez said. "He got stronger as the game went on. He did a terrific job."
With the rally in the late innings, the Marlins were able to complete their fifth comeback win of the season. Leo Nunez collected his fifth save, working around one hit in the ninth.
"Losing three in a row, and getting swept in a series, you want to start the next one off right," said Volstad, who limited the Nationals to three runs in 16 innings over his last two starts. "I wanted to do that for the team. I wanted to shut the Nats down, and help us get back on the winning side."
With the score knotted in the eighth inning, Petersen slapped a single to left off Brian Bruney. Chris Coghlan dropped a sacrifice bunt, and reached on Bruney's throwing error. Gaby Sanchez lined an RBI single to center, giving the Marlins a 3-2 lead. Cantu's sacrifice made it a two-run game.
"It's just good to get back on the winning side," Volstad said.
For most of the night, the Marlins were held in check by Craig Stammen, who struck out a career-high eight in 6 2/3 innings.
Scoring chances were slim for both teams most of the night.
Ryan Zimmerman's fifth home run of the season, and third off the Marlins, put the Nationals ahead, 2-1, in the sixth inning.
The Nationals manufactured a run in the first on Josh Willingham's fielder's-choice RBI groundout. Cristian Guzman, who had singled, scored when the Marlins were unable to complete a double play on Willingham's grounder to third.
In the second inning, Cody Ross's two-out RBI double pulled the Marlins even at 1. The run was unearned because Baker reached on Ian Desmond's error. Ross has hit safely in 10 straight games. John Baker hit his first career triple in the seventh, but he was stranded.
"This was huge," Ross said of the win. "It was a big win for us, especially after that homestand we had, which was awful. Everyone wants to get back on the right track, and tonight was a good start for us. Hopefully, tonight will do that."