Bryce Harper walks off the field with blood on his neck. (Lucy Nicholson/Reuters)

Monday night seemed close to perfect for the Washington Nationals, until it veered close to catastrophe. A blistering afternoon yielded to a warm Southern California night, Jordan Zimmermann further asserted himself as one of baseball’s elite starting pitchers, Ryan Zimmerman stayed hot and first place beckoned. And then Bryce Harper was there on the Dodgers Stadium warning track, lying still on his back as trainers rushed toward him and the ballpark hushed.

The Nationals thumped the Los Angeles Dodgers, 6-2, and survived a mighty scare. Harper walked off on his own power in the fifth inning after a frightening, face-first collision with the right field fence that left him bloodied and the rest of the Nationals startled.

Harper received 11 stitches under his chin but did not suffer a concussion, according to Nationals officials and Harper’s agent Scott Boras. Harper also underwent “precautionary” X-rays on his jammed left shoulder, Manager Davey Johnson said. He also bruised his neck and knee. The prevailing sentiment was that Harper would probably miss a game or two — even if he pleaded to stay in Monday night as blood dripped down his neck.

“Bryce is going to be all right,” Johnson said.

Harper’s nasty crash into the outfield wall prevented the Nationals from fully celebrating the latest gem from Zimmermann, who is making a routine out of mastery. He allowed two runs over 72 / 3 innings, striking out five Dodgers and scattering nine hits as he became the first pitcher in the majors to reach seven wins. Zimmermann carved through the Dodgers’ lineup, daring them to hit his 95-mph fastballs, mixing in just enough biting sliders, always coming straight at them.

Paced by Zimmerman’s two-run, bases-loaded double, Zimmermann settled his ERA at 1.69. Since the start of the 2012 season, the only pitchers with an ERA better than his 2.65 are Clayton Kershaw (2.35) and Justin Verlander (2.52).

“I’ve always been confident,” Zimmermann said. “I feel like I haven’t changed anything all year, or changed anything from last year. It’s just the way the ball is bouncing right now. I’m pitching to contact, throwing strikes. If I keep that up, I’ll have more quality starts.”

Most nights, Zimmermann would have again held top billing. As Monday turned into Tuesday back east, though, the Nationals had to worry about the condition of their best player.

Leading off the fifth inning, Dodgers catcher A.J. Ellis roped a line drive over Harper’s head in right field. Harper sprinted after the ball looking over his left shoulder, even as he reached the warning track. As the ball crashed off the wall, Harper turned, apparently unaware of his proximity to the fence. He smashed face-first into it like he never saw it coming.

“I thought he was going to try to brace himself or do something,” center fielder Denard Span said. “As the ball kept going, I just was like, ‘Is he going to stop?’ And he just kept going.

“It just looked weird, like it caught him by surprise the ball was hit to him,” Span added. “The way he ran into the wall, he definitely had no idea where he was. As soon as he ran into it, it’s like his body locked up. I’ve never seen anybody run into the wall like that.”

Harper’s head snapped back and his hat came flying off his head like a piece of ski equipment after a crash. He crumpled to the warning track dirt. Dodgers Stadium quieted. Harper lay still as Span picked up the ball and flung it into the infield, not even looking where the ball went.

Before play had stopped, head athletic trainer Lee Kuntz had started hustling out to right field, Johnson behind him. Span tried to relax Harper as they jogged.

“Initially, it was like he was confused,” Span said. “I don’t think he realized he was on the ground. I think once it set in that he was on the ground, he was okay. And he just kept asking me, ‘Is it bad? Is it bad?’ I’m like, ‘You’re bleeding a little bit.’ ”

Before they arrived, Harper started writhing and rolling over, Span standing over him. Kuntz inspected him, and after a minute Harper rose to his feet, blood trickling on his neck. He had run into the lighted scoreboard portion of the fence, which may have accounted for the cut.

Harper, who had a single, two walks and two runs in three plate appearances, tried to persuade Johnson and Kuntz to let him stay in the game. But he walked off the field with them, the blood forming like a necklace under his chin the only noticeable damage.

“It definitely wasn’t a pretty sight, seeing the blood trickle down his neck,” said Span. “Thank God he was able to get up. He actually was trying to stay in the game. I was looking at him like, ‘No, you need to come out of the game.’ He’s a warrior. I guarantee he’s probably going to try to play tomorrow. I just thank God he’s okay.

“He kept telling Davey, ‘I’m okay, serious.’ I was like, ‘Is somebody going to step up and say he’s not okay? Because he doesn’t look too good.’ ”

A collision between Harper and the wall already scared the Nationals once this season. On April 30 in Atlanta, Harper badly bruised his left side as he leaped at the fence to try to steal a home run. He left the next night’s game after six innings, but returned the next day. Harper went 2 for his next 19, clearly affected by the ailment.

“He’s not worried about the wall or anything,” Johnson said. “He should know it’s on the warning track and back off, but that’s not his nature. I don’t want to change that. I feel sorry for the wall if he keeps running into them.”

Monday night, as Roger Bernadina replaced Harper in left, the Nationals were still holding their breath as they finished off the Dodgers.

Zimmermann’s crisp aggression stood in contrast to Dodgers starter Josh Beckett’s glacial nibbling. Beckett, one of the Dodgers’ many big names producing small results, held the ball as if afraid it would not come back if he threw it. In the first inning alone, Beckett took 17 minutes to throw 25 pitches. Zimmermann could have been forgiven if he asked to play through.

“Beckett was going so slow,” Zimmermann said. “And I was trying to go as fast as I could to get back on pace.”

The Nationals stayed awake in the batter’s box long enough to snare an early lead. Zimmerman scored Span with an RBI grounder in the first. In the third, they pulled away. Harper drew a walk to load the bases, and Beckett learned that Zimmerman will make pitchers pay for avoiding Harper. He smoked a two-run double off the right field fence, making him 9 for his last 25.

“It takes a couple days to get back,” Zimmerman said. “But I’m starting to get back into the groove now, and I’m starting to feel better at the plate.”

Adam LaRoche took advantage of Dodgers Manager Don Mattingly’s curious decision to play the infield back. His routine hit to first base scored the Nationals’ fourth run rather than stalling the rally. The Nationals had seized a four-run lead, more cushion than Zimmermann has needed all season, and Mattingly yanked Beckett after just 68 pitches and three innings.

The game devolved for the Dodgers during a two-run fifth inning. Steve Lombardozzi’s chopper snuck by both middle infielders, and then reliever Javy Guerra walked two batters and chucked a potential double-play ball into center field.

The Nationals jogged back to the dugout, at ease with a victory well in hand. They could have pulled even with Atlanta for first place had the Braves not won in Arizona. And then, as Harper hit the wall and fell to the dirt, they had something much bigger to worry about.

“Anytime that kind of stuff happens with anyone, it’s no good,” Zimmerman said. “Him walking off being alert obviously makes it look a little bit better. Hopefully, he’s okay.”