(Mark J. Terrill/AP)

Harper takes every at-bat prepared, with a specific plan for the pitcher and the situation. It doesn’t guarantee him success, obviously, but he has a maturity at the plate you would not expect from a 19-year-old with about 600 professional plate appearances. When he makes his Nationals Park debut tonight, against Diamondbacks right-hander Trevor Cahill, Harper will go to the plate not just to react, but with a clear idea in his head.

Hitting coach Rick Eckstein identified Harper’s advanced approach during spring training. He had a conversation with Harper about a specific National League East pitcher. Having just watched him on television, Harper laid out how he expected that pitcher would attack him. Eckstein nodded and told him he agreed.

“There are so many things that go through my head when I’m watching something,” Harper explained during spring training. “I’m a little different. I sit there and think. I’ll watch it as a fan, but I’m a player, also. If everybody is looking cutter first pitch, and he’s throwing a get-me-over curveball 90 percent of the time, then you got to sit on that get-me-over curveball and crush it.”

In the ninth inning of his debut, Harper came to the plate with a man on third and one out, the go-ahead run 90 feet away. He simplified his approach and looked for a pitch he could whack into the outfield to score the run. When Javy Guerra threw him a first-pitch fastball, he was ready. He drove it to the opposite field and gave the Nationals’ the lead.

He came up in a different situation in the ninth Sunday. The Nationals trailed by two with two outs in the ninth with a man on first. Harper had a specific aim: He would try to crush a ball and tie the game, or he would take a walk from hard-throwing Kanley Jansen and bring the go-ahead run to the plate.

I wanted to hit a bomb, for sure,” Harper said. “He gave me two pitches I could drive, and I fouled both of them back. He throws pretty hard, has a really good sinker. He kept on going sinker, sinker, sinker. I just tried to get something I could drive, and if I didn’t, I was going to draw a walk.

”I was just trying to get something elevated, the inside half or even over the middle so I drive it to center or something like that. He supplies all the power, of course. He’s throwing 95 mph. The second swing I took, I got out on my front foot twice. I tried to stay back on that the last 4-5 pitches.”

Harper ended up fouling back on 3-2 pitch and then taking a closer sinker just off the outside corner. Even while anxious to find a pitch he could clobber, Harper showed the patience and discipline to not chase three sinkers just off the plate.

Harper showed some pretty good discipline yesterday on the Mall, too, while jumping into a slow-pitch softball game: He caught the first pitch lobbed to him. He did whiff on his first attempt.

Harper was driving by the Mall, spotted the game and asked to jump out and play. How cool is that?

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