SAN FRANCISCO -- Nationals first baseman Adam Dunn thought he hit his second home run of the game in a 5-4 loss to the Giants at AT&T Park on Thursday afternoon, but it was ruled a double.

With the Nationals leading, 3-2, Justin Maxwell on first and one out, Dunn took a pitch from Giants left-hander Barry Zito and hit a ball that he thought was over the right-center-field wall for a home run. But the ball hit the white cement over the State Farm sign and second-base umpire Paul Nauert ruled that the ball was still in play.

"I hit it good enough to be a homer," Dunn said. "I wasn't sure if it was going to be a homer or not, and it wasn't."

Manager Jim Riggleman argued with first-base umpire Casey Moser that Dunn hit his 11th home run of the season. Moser and the rest of the umpiring crew huddled and then looked at the replay.

After a brief review, the call stood. Riggleman said he agreed with the umpires' decision.

"The ground rules were clear before the game. If it hits that white area, it's in play," Riggleman said.

Said Giants manager Bruce Bochy: "I didn't think it was out. We had a break there because it was probably a foot away -- I don't know how far away it was from being a home run -- and that would have been a couple more runs. So we had a break there, and it kept us in the game close enough to where we could come back."

Aaron Rowand, who has been the Giants' center fielder since 2008, said the home run controversy has rarely surfaced during his time with the team. In fact, Rowand said even he isn't aware of what the rule is for home runs in the right-center field area, but appreciated the umpiring crew taking the time to get it right.

"They got the call right the first time and went to check on it to make sure and it turns out they were right, so that's good," Rowand said.

The call turned out to be one of the turning points of the game. Bochy then decided to walk Ryan Zimmerman intentionally to load the bases with one out. But the Nationals were not able to break it open even though they scored one run on a sacrifice fly by Josh Willingham. After Willingham's at-bat, Ian Desmond struck out looking to end the inning.

"It did [hurt not to add on runs]. It's something you have to do," Riggleman said. "You have to add on when you can. We did what we could do. We got the one, but we would have liked to have added a little more. It's the ground rules."