MIAMI (AP) Wander Suero was trying to pick a runner off first base when he let another stroll into home.
Washington led 4-0 after 4 1/2 innings but trailed by the end of the sixth. Suero let in the go-ahead run, balking with pinch-hitter Rafael Ortega batting and Austin Dean at third base to put Miami ahead 5-4. Suero was trying to pick Magneuris Sierra off first, and Nationals manager Dave Martinez was uncertain what he did wrong.
''He (home plate umpire Pat Hoberg) said he saw something different,'' Martinez said. ''He's been doing it all year. It's the first it's ever been called a balk.''
Washington tied it in the seventh when Trea Turner beat out a potential double play, but Miami loaded the bases in the bottom of the inning and Lewis Brinson hit a tiebreaking single off Koda Glover (1-3).
The Nationals (76-75) are in third place in the NL East, trailing the division-leading Braves (83-67) by 7 1/2 games and are 6 1/2 games out of the final wild-card spot.
''We have to win every one,'' Martinez said. ''Let's concentrate on tomorrow, but we have to win these games. We're still in it and I know those guys are not going to quit. Forget about this one and come back tomorrow and win tomorrow.''
Anthony Rendon had three hits, including a two-run home run, and Victor Robles hit his first career homer for the Nationals, who have 11 games remaining.
''You obviously feel bad because you still have a chance to qualify for the playoffs,'' Robles said.
Starlin Castro had a homer and three RBIs for the last-place Marlins.
Castro's solo homer in the eighth pushed the lead to 8-5.
Jarlin Garcia (3-2) recorded the final out of the seventh. Adam Conley pitched the ninth for his third save.
''A lot of good things today,'' Marlins manager Don Mattingly said.
The Nationals led 4-0 behind an error by Marlins third baseman Brian Anderson, a solo shot by Robles and a two-run blast by Rendon.
Castro's two-run double in the fifth cut the deficit to 4-2 and ended Erick Fedde's outing. Fedde allowed two hits and two runs over 4 1/3 innings.
''He hits that double and it really gets us back in the game,'' Mattingly said.
Miami got four consecutive hits off Jimmy Cordero in the sixth, including a two-run single by Sierra, before Suero balked.
''You try to do your job, make good pitches and locate, but things happen in games,'' Cordero said. ''You just have to keep attacking. You can't let it affect you because you lose control.''
Marlins starter Trevor Richards allowed four runs in five innings.
Marlins: RHP Jose Urena (hand) exited Sunday's game at Philadelphia but is expected to be fine. Manager Don Mattingly said he was just experiencing cramping in his hand. ... RHP Dan Straily (oblique) is not expected to pitch the rest of the season.
Nationals: RHP Stephen Strasburg (8-7, 3.87 ERA) is the scheduled starter in Tuesday's game against Miami to end the two-game set.
Marlins: RHP Sandy Alcantara (2-0, 1.42) will make his fourth start of the season and third against the Nationals.
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