MIAMI -- Enny Romero's neck whipped around in an instant.

The Washington Nationals reliever had just thrown an 0-2 fastball that was clocked at 97 mph on its way to the plate before making contact with the bat of Marcell Ozuna, the Miami Marlins left fielder who is having an All-Star-caliber season.

Ozuna rocketed that pitch into the gap in left-center field for a bases-loaded, two-out, walk-off single in the ninth as Miami rallied from an early six-run deficit and defeated Washington 8-7 on Monday night at Marlins Park.

"That's sweet," said Ozuna, who leads Miami with a .327 batting average. "I said, 'OK, stay short and put the ball in play,' and that's what I did."

He did quite a bit more than that, and the Marlins (31-37) twice tied Washington with homers -- a grand slam by Justin Bour in the third and a solo shot by Giancarlo Stanton in the seventh. It was the 18th homer of the season for both Stanton and Bour, tying them with Ozuna for the team lead.

Washington (42-28) was led by Bryce Harper, who also hit his 18th homer, and Anthony Rendon, who hammered his 13th before leaving due to a neck stinger. He is listed as day-to-day.

Miami's winning rally started with a two-out, four-pitch walk drawn by Dee Gordon, who stole second base for his 27th swipe of the season.

Gordon advanced to third on a Stanton infield single that bounced off the knee of Nationals second baseman Daniel Murphy, who could have ended the inning had he fielded the hard shot cleanly.

That became a huge play when Ozuna hit his shot that was ruled a single because that was all that was required to earn the win.

It was sweet redemption for the Marlins, who lost on walk-off hits by the Atlanta Braves' Brandon Phillips on Saturday and again on Sunday.

Miami's bullpen was strong on Monday, however, as five Marlins relievers combined to allow just one run in six innings. A.J. Ramos (2-3) pitched a scoreless ninth to earn the win.

Romero (2-3) took the loss in another rough night for a Nationals bullpen that is the worst in the league in numerous categories.

"These guys have been giving up runs," Marlins manager Don Mattingly said of Washington's bullpen. "When you get into their bullpen, you never know what's going to happen ... but you know you have a chance to score some runs."

As for the starting pitchers, neither lasted long in the first meeting of the two clubs since the season's opening series in Washington in April.

In his first start since returning from the disabled list due to a finger injury, Miami left-hander Justin Nicolino worked just three innings. He gave up Harper's two-run homer in the first inning and Rendon's solo shot in the second.

Washington built the lead to 6-0 in the third on a run-scoring double by Murphy and a two-run single by Michael A. Taylor.

However, the Marlins immediately battled back against Nationals starter Tanner Roark, who beat Miami on April 5.

This time, Miami scored six times in their half of the third. J.T. Riddle tripled in Derek Dietrich and later scored on Gordon's RBI groundout to trim the deficit to 6-2.

Bour then tied it 6-6 by clubbing his third career grand slam, launching a 2-2 changeup from Roark into the right-field bullpen.

"You get a six-run lead and give it up -- that's pathetic on my part," Roark said. "I felt good, I just pitched bad.

"This one hurt, but it's a long season."

Despite Roark's poor performance, Washington's offense kept up the pressure in fifth. Stephen Drew, who replaced Rendon midway through the game, doubled to score Ryan Zimmerman, giving the Nationals a 7-6 lead.

"Drew came in and did a great job," Washington manager Dusty Baker said after Rendon was hurt diving for a ground ball.

Stanton tied the score 7-7 in the seventh by hitting an opposite-field homer off of former Marlins pitcher Jacob Turner, setting the stage for the dramatic conclusion.

"This was a great game to be a part of," Bour said. "We were down six runs but never really out of it. This is a great time to be a Marlin. We're having fun."

NOTES: Miami optioned RHP Drew Steckenrider (0-1, 5.40 ERA in four games) to Triple-A New Orleans. ... Marlins 3B Martin Prado (hamstring) and SS Adeiny Hechavarria (oblique) are expected to return to the majors after a few more minor-league rehab games. ... Washington's 3-4-5 hitters in Monday's lineup -- RF Bryce Harper, 1B Ryan Zimmerman and 2B Daniel Murphy -- all lead NL All-Star voting at their positions. ... Of Harper's 18 homers, just two have come against left-handers, including one on Monday against Justin Nicolino. ...Nationals LHP Gio Gonzalez will start on Tuesday against his hometown team. Gonzalez, a native of Miami Dade County, is 1-2 with a 4.24 ERA in four career starts at Marlins Park. ... Three other Nationals players have South Florida ties: OF Brian Goodwin played at Miami Dade College, CF Michael A. Taylor grew up in Fort Lauderdale, and SS Trea Turner is from Boynton Beach.