Nationals lose to Marlins 3-2 on homer in 9th
The Washington Nationals had hoped to finish the season with a winning record. Now, they won't even make .500.
Petersen hit the first pitch from reliever Doug Slaten (0-2) for the Marlins' third walkoff homer this season.
The loss ended the Nationals' bid for the franchise's first winning season since 2003. They are 79-81 with one game left.
''We've made a lot of progress,'' left fielder Michael Morse said. ''There's a lot of bright futures ahead for a lot of guys.''
Morse hit his 31st home run and will likely sit out the season finale Wednesday, leaving his batting average at .303.
''I feel like I've achieved a lot,'' Morse said. ''I've put in a lot of hard work and am glad to see it paid off.''
The 35-year-old Vazquez (13-11) may have pitched the final game of his career as he is likely to retire at the end of the season.
''I've been blessed being in the big leagues for 14 years,'' Vazquez said. ''I feel it's time. I'm glad I'm pitching well because it would be tough retiring on a bad note.''
The Marlins also had only five hits, but Petersen's home run was the one Vazquez needed.
''Any home run I hit is cool for me, but it's way more special for me to know that I be a part of that for him,'' Petersen said. ''I hope that game sticks with him for the rest of his life if it's his last one.''
Nationals starter John Lannan went six innings and gave up two runs and three hits, striking out four.
Washington went ahead 2-1 in the fifth. Steve Lombardazzi opened the inning with a double. After advancing to third on a sacrifice bunt, he scored on a single by Ian Desmond.
Florida tied it at 2 in the bottom half when Emilio Bonifacio doubled, stole his 40th base of the season and scored on a wild pitch by Lannan.
''They're busting their butt out there to get you a lead, and you blow it like that - it's not a good feeling,'' Lannan said. ''It's not easy back there when I'm missing my spot by five feet.''
Vazquez's streak of 29 scoreless innings, a Marlins' club record, was broken when Morse led off the fifth with a solo homer to right-center that tied the score at 1. The previous Marlins' record was 26 1-3 innings set by Luis Aquino from May 9-July 26, 1994.
''We went up against a great pitcher who pitched a heck of a ballgame,'' Nationals manager Davey Johnson said. ''We only got five hits. I like the effort my guy gave. He was cruising along until the sixth and for some reason got a little wild.''
The Marlins took a 1-0 lead in the third when Matt Dominguez led off with a double, advanced to third on a sacrifice bunt by Vazquez and scored on a sacrifice fly by Bonifacio.
Wednesday's game will be the last in the stadium that has been the Marlins' home since their first season in 1993. Next year they move into a retractable-roof ballpark in downtown Miami.
NOTES: Nationals 3B Ryan Zimmerman sat out with a sore hamstring, and it's unlikely he will play Wednesday. ... Nationals RHP Stephen Strasburg makes the fifth start of his comeback from elbow surgery Wednesday. He's 0-1 with an ERA of 2.00 in 18 innings. ... RHP Chris Volstad, who will start for Florida, has an ERA of 2.65 in his past six starts.