Zimmerman connected on a 1-2 pitch off Miami's Nathan Eovaldi and crushed a deep drive to center field. The ball bounced off the top of the 13 1/2-foot wall, deflected off the base of the home run sculpture, and returned to the field of play.
Not having a clear view, the umpires didn't signal home run, and Zimmerman stopped at second with what was initially ruled an RBI double.
Immediately, Zimmerman made the hand motion for a home run, and Nationals manager Davey Johnson asked the umpires to check the replay.
Crew chief Gary Cederstrom huddled with the umpires, and they trotted off the field for a review.
A couple of mintues later, the double was overturned to a two-run homer.
Denard Span opened the game with a single to left, and Zimmerman followed with his two-run shot.
For Zimmerman, the homer was the 20th of his career against the Marlins, his most off any team. He followed with his 21st home run against Miami in the third inning, a solo homer.
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro. This story was not subject to the approval of Major League Baseball or its clubs.