A look back at Harper’s first 18 games over his previous four seasons shows that he has surpassed his early performances in 2012 (.227/.316/.439 with two home runs ), 2013 (.353/.421/.706 with seven home runs), 2014 (.279/.347/.397 with one home run) and 2015 (.271/.429/.492 with four home runs).

“I’ve played with some really good players but no one that’s done what he did last year and then followed it up with the start to the season like this,” said veteran outfielder Chris Heisey, who hit Sunday’s walk-off home run in the 16th inning thanks to, in part, a hitting drill Harper showed him. “It’s really incredible just to watch him do what he’s doing. It’s a joy to watch, and I try to learn as much from him as I can. …

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“We come to expect that he’s going to do something special. You can’t not expect it when he’s been doing it as consistently as he’s been doing it. I find myself putting really unfair expectations on him. If he gets jammed or pops out, it’s like ‘What the heck, Harp?’ You have to remember that it’s very hard. I’m a career .240 hitter.”

The at-bat in the ninth inning of Sunday’s wild game, which led to the pinch-hit blast and seven more innings of play, was vintage Harper. He fouled off would-be strikes he didn’t like and took balls, including one just outside the outer edge. Then he crushed a low 96-mph fastball over the middle of the plate, in his sweet spot.

“He believed that he was going to do it,” Manager Dusty Baker said. “That’s what impressed me the most. Confidence is not his problem. You know what I mean? Anything he does, he doesn’t seem surprised, and I’m not surprised. But I’m extremely happy, and I’m sure he is, too.”

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Harper has homered nearly every seven at-bats this season, a rate that would rival Barry Bonds’s 73-homer season in 2001. Bonds did that over 153 games; Harper has played just 18 this season. But Harper has succeeded when hitting the ball in the air this season, which he is doing more.

According to Baseball-Reference.com, a quarter of Harper’s fly balls have resulted in home runs, a higher rate than his 20 percent rate in 2015, when he totaled 42 home runs. When Harper put the ball in play last season, he had a .369 average. So far this season, he has a .239 average on balls in play, perhaps a combination of bad luck and hitting into the defensive shift. So hitting the balls in the air — and over the fence — has worked.

“I really just try to have good at-bats,” Harper said of his hot start. “Try to have the right approach of getting on base and really just trying to swing at the pitches I can and definitely be more patient, get better at that. Swinging at pitches out of the zone a little bit more the past couple games but trying to make things happen. Just trying to stay within myself, trying to get a pitch I can drive.”

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After his pinch-hit home run Sunday, Harper watched the game with his teammates from the bench. During the 14th inning, they donned rally caps, turning their Nationals hats inside out or sideways on their heads. Jonathan Papelbon wore a helmet and sunglasses. Even as the game dragged into its fifth hour, Nationals players laughed, cheered and kept plugging along for the win.

“This team is very special, and it’s a lot of fun to be a part of it and watch and it and just help a little bit,” Harper said.

Harper reserved some of his highest praise for Baker. The star outfielder has said before that he has liked every manager he has played for, including Matt Williams last season. Harper, though, especially loved how every button Baker pushed tactically Sunday worked. And he loves that Baker has fostered a team culture where players have fun and can be themselves, including wearing hats sideways during a wild back-and-forth game.

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“Baseball at every single level, and if you’re in high school, college, Little League anything, that’s fun right there,” Harper said. “And to be able to have the opportunity to play for Dusty, that desire and that mentality that he brings every single day to let us just have fun to let us enjoy this game, with all the rally caps and all the stuff we were doing and just going crazy and everything you could imagine. And he just lets us play, and that’s what the game is all about.