Bryce Harper hitting a double off Jeremy Affeldt. (Photo by Greg Fiume/Getty Images)

The scary thing right now about the Washington Nationals, who have seized the best record in the National League by winning 12 of their past 13, is that they may be closing in on an even higher ceiling: If Bryce Harper produces at his highest level, their lineup will become even more dangerous. And Harper showed signs of a breakout over the past 10 days.

As the Nationals went 9-1 on their homestand, Harper went 12 for 37 (.324) with one double and one homer, both extra-base hits coming yesterday. “It felt pretty good,” Harper said. “I’m just trying to have good A-Bs. I had a good homestand, so I was happy.”

On paper, Harper’s homestand totals do not stick out. But the kind of at-bats he took suggest he’s approaching full capacity. Harper peppered center field with line-drive singles, staying back on pitches and ripping them up the middle.

On Sunday, in the pivotal sixth inning, the Giants brought in sidewinding lefty Jeremy Affeldt, who last year held opposing lefties to a .196 average. He is the kind of pitcher who chewed up Harper last season, when Harper’s knee injury affected his swing in a way that made him vulnerable to offspeed pitches breaking away from him. This year, with a healthy knee, Harper is hitting .284 with a .759 OPS.

“I actually don’t mind facing lefties,” Harper said. “I’ve been saying that for three years now. Being able to be healthy with my knee, being able to go up there and really take pitches and feel what I want to feel with my backside against the lefties is nice. Being able to go up there with the confidence I have, Affeldt is pretty dang good.”

And Harper beat him. Affeldt buckled Harper with a first-pitch curveball, then complemented it with a sinker away. Harper waited, stayed with the pitch and drilled a line drive to left field. It became a double when Michael Morse couldn’t chase it down, but even if a better left fielder had made the play, the point stood: Against a tough pitcher making a tough pitch, Harper smoked one.

In the eighth inning, Harper’s power showed. Juan Gutierrez, a right-hander and the last pitcher in the Giants bullpen, threw him a high-and-inside fastball. Harper mauled it with a swing out of April 2013, the ball vaporizing at contact with a bat moving with cartoonish speed.

In the dugout, Manager Matt Williams associated the home run with all of those line drives to center field from earlier in the road trip.

“He’s hit  a lot of balls back through the middle in recent days,” Williams said. “It tells me that he’s seeing it and he’s staying back. The ball to left is especially important for him. Then he got a ball up, middle of the plate, that he was able to hit for a homer. It’s funny how those come when you’re hitting the ball the back through the middle, and it’s the case with everybody.

“You see it with [Ian Desmond]. You see it with Adam [LaRoche]. When he starts hitting the ball back through the middle and the opposite way, then he’s able to stay on the curveball, and those balls go in the seats. It’s the same with Bryce. If he hits the ball back through the middle consistently, he’ll get pitches to hit where he can pull them and be able to hit a ball over the fence.”