(Julio Cortez/AP)

Today, Harper went 1 for 4 with a line-drive, first-pitch single to right off Kris Medlen in his first at-bat, probably the hardest ball he’s hit in a game this spring. He is 5 for 11 with a walk this spring.

Harper’s most notable play, though, came on the base paths. With one out, Harper stood on first base when Wilson Ramos hit a dribbler to third. As Martin Prado charged and whipped the ball across the diamond, Harper bolted around second base. He paused for a moment, then started again toward third.

Freddie Freeman threw the ball back across the field, and Harper fell over trying to stop himself. Prado tagged him out to end the inning.

“He wanted to be aggressive, and then there was a little hesitation, and he realized, ‘Oh, I’m not going to make it,’ ” Nationals third base coach Bo Porter said. “I said, ‘Your aggression, I don’t mind. If you’re going to be aggressive, just go ahead and be aggressive.’ Just commit and go. If you’re out, you’re out. It lets you know your limitations or what you can do.”

Harper’s hustle Monday night stood out. On one play, he sprinted from first to third on a groundball that got through the left side of the infield. The Nationals have no issue with Harper’s base-running abandon, but today Porter wanted him to simply make a choice and commit.

“It’s an okay play if you don’t hesitate,” Porter said. “Once you hesitate, then you just go ahead and you stay. If he was aggressive all the way, and then he’s out, you love the aggression, you go, ‘They made a play,’ and he’s out.”

Afterward, Porter pulled Harper aside and explained another layer to the play. Harper batted seventh, and Ramos batted eighth. So, in a normal National League game, the pitcher would have been due up next. (At this stage of spring training, designated hitters are used.) Porter told Harper that he would have to be mindful on the bases of those situations — he would have to be conservative, to avoid forcing the pitcher to lead off the next inning.

“He understood completely,” Porter said.

The Nationals’ favorite part about Harper’s game this spring may be hustle. They understand mistakes like today’s will come on the bases, but in some ways that may be better to take than the alternative.

“I’m glad I don’t have to gear him up,” Manager Davey Johnson said. “It’s much easier to gear somebody down than have to ramp them up, and he’s ramped up.”

● When Mark DeRosa arrived at spring training a few weeks ago, he expressed confidence that his left wrist, repaired three times by surgery, would hold up this year. His performance at the end of last season assured him, but even then, of his 18 hits in 57 at-bats, only one went for extra bases. He still had some doubts if he could hit for power.

“That is the name of the game, driving in runs and being a dangerous hitter and occasionally driving the ball out of the ballpark,” DeRosa said then. “I hope to get back to that.”

Today, then, offered an encouraging sign. DeRosa smashed a home run to left field, into the wind, off Jonny Venters, one of the hardest-throwing relievers in baseball. DeRosa hit one home run last spring training, but before that, between injuries and ineffectiveness, DeRosa had not hit a home run since April 5, 2010.

“That was great, but it was even more when he came in the dugout, his confidence,” Johnson said. “He had everybody laughing. He was talking about, ‘Whoa! Wow! I’m back!’

“He’s been swinging the bat great. DeRosa, he’s been talking about he doesn’t belong on this club with all these big guys hitting home runs. That was a bullet.”

DeRosa could become an important player for the Nationals, who plan to alternate him between first base and in right field against left-handed starters. The Nationals put a lot of faith in his ability to stay healthy and provide right-handed pop off the bench. Today was a positive sign.

Jayson Werth also hit his first home run of the spring, a laser that rode the wind to right field and bounced off the top of the fence. Werth creamed the first pitch Medlen threw him. “I mean, when has he ever swung at a first-pitch fastball?” Johnson asked.

●Ian Desmond made another error today, his third of the spring. He bobbled a routine groundball, unable to make a clean transition from his glove to his hand. His start to spring has not been helped, either, by the fact that he has failed to reach base in his first nine plate appearances.

“He sometimes, I think, might take his at-bats to the field,” Johnson said. “Especially down here. You’re concentrating on getting your timing. But I thought he swung the bat better today. The [defense], I’m not that concerned with it right now.”