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NEW YORK -- Josh Willingham saw the ball carom off the outfield fence and put his head down, charging hard around first base. He kept going past second and was waved around third, plowing into Mets catcher Rod Barajas at home plate.

It sure was a lot of wasted effort.

Willingham's bases-loaded drive off Johan Santana was initially ruled in play, but the umpires overturned it with some help from instant replay, giving Willingham a grand slam and propelling the Washington Nationals to a 5-2 victory over New York on Sunday.

"When you're facing Santana, you're not going to have many opportunities," Willingham said. "It was big to get some runs off him early, because you know you won't get many."

Willingham followed his fifth career slam with an RBI double in the third inning, helping the Nationals beat Santana (1-1) for the first time since June 9, 2007.

Livan Hernandez (1-0) pitched a gem against his former team, scattering five hits and three walks over seven innings. He needed only 88 pitches before giving it over to the bullpen, which surrendered a two-run homer by Mike Jacobs in the eighth before finishing up.

"It's kind of disappointing in that it seems that we weren't able to solve Livan," Mets manager Jerry Manuel said. "Johan's going to have an inning here or there. It would have given us a good lift to fight our way back into the game, but we weren't able to do it."

Matt Capps worked a perfect ninth inning for his third save, allowing the Nationals (3-3) to reach .500 for the first time since April 5, 2008. They took two of three from the Mets to win their first series at year-old Citi Field.

Hernandez was considered a long shot to make the team after arriving at Nationals camp in February, and the portly pitcher worked hard to get into shape. He allowed only two earned runs in eight innings in two spring starts, helping him earn the last spot in the rotation.

"It was my first game. I just tried to get people out and keep them off-balance," Hernandez said. "It's difficult to come back [in] the first game you're pitching."

Washington tried to make things easier during its frenetic first inning.

Santana had loaded the bases with a leadoff triple by Nyjer Morgan and walks to Willy Taveras and Adam Dunn, before Willingham drove a 2-1 pitch deep.

"I had gotten behind in the count," Santana said. "I threw him a changeup that didn't do anything and he put a good swing on it."

The ball bounced off the wall into center and the relay throw eventually got to Barajas, who was plowed over by Dunn trying to score. The ball squirted loose and Santana, backing up the play, tossed it back to Barajas just before he was run over by Willingham. This time, the Mets catcher managed to hold onto the ball for the out.

Not that any of it mattered.

Nationals manager Jim Riggleman immediately argued the play should have been a grand slam, and the umpires conferred for a moment before heading to the replay booth.

The top of the outfield wall at spacious Citi Field is about twice as high in left field as in center. The orange line painted along the top to signify a home run is vertical where the two sections meet, causing some confusion about whether the long drive was in play.

Replays showed it hit just to the right of the vertical line and over the horizontal line, and crew chief Derryl Cousins quickly ruled it was a home run.

"If the umpire gets it right the first time, I don't have to have these bumps and bruises," Barajas said with a straight face.

Major League Baseball began using replay in August 2008, following the NFL, NBA, NHL, some NCAA competitions and Grand Slam tennis. Replay can only be used on home runs involving boundary calls, such as whether a ball was fair or foul, or whether it cleared the outfield wall.

Game notes

The benches and bullpens cleared in the ninth when Mets reliever Francisco Rodriguez hit Willie Harris with a pitch. The two exchanged harsh words but no punches were thrown. "I don't think he hit me intentionally, and if I said anything, it wasn't directed at him," Harris said. "If I thought it was intentional, I would have gone after him right there." ... Washington 3B Ryan Zimmerman missed the game with a tight left hamstring. ... The Mets finished their six-game homestand 2-4 and now play six on the road, beginning Tuesday at Colorado.