Tanner Roark allows seven hits and two earned runs over six innings to earn his fifth win, but he received a spectacular assist from fledgling left fielder Ryan Zimmerman to close out the sixth inning. (Ezra Shaw/Getty Images)

Appointed earlier Wednesday evening as a short-time left fielder, Ryan Zimmerman watched the ball slice off Brandon Crawford’s bat and sail into the left field corner at AT&T Park. Tanner Roark stood on the mound, hoping another stellar start would remain unblemished. Michael Morse danced off second base, where he had reached on account of Zimmerman’s arm. Zimmerman reacted immediately, sprinting to his right.

“Just go for the ball, I guess,” Zimmerman said. “I’m kind of learning on the fly.”

Other developments were more responsible for the Washington Nationals6-2 victory over the San Francisco Giants, beginning with Giants starter Matt Cain walking the first three batters of the game. Other moments led to the Nationals’ 10th win in 12 games, including Jayson Werth’s first home run in nearly a month, his two-run single in the ninth and Roark’s six strong innings. But no play stirred more drama, or whipped the Nationals’ dugout into a wilder frenzy, than the play Zimmerman made to end the sixth inning.

“It’s remarkable, really, to see him change positions and look so natural,” Werth said. “It’s not beginner’s luck or anything. He’s getting good reads. He’s running good routes. I said the other day, he makes left field look small. As an outfielder, that’s the ultimate compliment.”

Zimmerman’s diving catch, his best moment yet in left field, helped the Nationals claim their four-game series against the Giants before Thursday’s finale. The Nationals also seized sole possession of first place in the National League East after the Atlanta Braves fell in Colorado. The Nationals arrived at AT&T Park to face the best team in the National League. If they can pull off a sweep Thursday — their record aside — they may leave for St. Louis with the title attached to them.

Blurb: With the starting pitchers turning in solid performances and the offense clicking, the Post Sports Live crew looks at whether the Nationals' road trip to San Francisco and St. Louis will be a good barometer for the team's status as a contender. (Post Sports Live/The Washington Post)

“It’s a tough place to come play,” Zimmerman said. “They’re the best team in baseball. I wouldn’t say it’s significant. But we try to win every series. We knew this one was going to be especially tough to win. To come in here and take the series, we’re playing good baseball. I don’t think we need to get caught up in it too much. But I think we should be proud of how we’ve come in here and played so far.”

In their latest victory, Zimmerman saved a run and scuttled one of the Giants’ best chances to inch back into the game. Morse had reached scoring position when he stretched a single into a double, testing Zimmerman’s arm — and winning when Zimmerman’s throw one-hopped cutoff man Ian Desmond.

But now Zimmerman closed on Crawford’s fly to left field. In the evening, Manager Matt Williams had reiterated that Zimmerman will head back to third base once Bryce Harper comes off the disabled list in early July. “I don’t think there’s much of a choice,” Williams repeated after the game.

Until then, Zimmerman will continue to play left, a position he has grown increasingly fond of.

And increasingly good at. Zimmerman laid out, stretched his backhand and snared the ball. As he slid on his stomach, Zimmerman’s face smashed into the grass. “The landing wasn’t very smooth,” Zimmerman said. “But in the end, I was just happy I caught the ball.”

The Nationals dugout erupted. On the mound, Roark screamed and pumped his fist.

“Unbelievable,” Roark said. “It was a huge play. I saw the ball kept slicing away from him. He’s a hell of an athlete, anyway. He’s quick out there. He gets good reads on the ball. It’s pretty impressive, actually.”

In the next inning, Zimmerman made another clutch play with a smaller degree of difficulty. The Giants knocked Roark (5-4) out with two singles, one a bunt and the other a seeing-eye grounder to right. Drew Storen entered to sweep up the mess, and he induced a double play from Joaquin Arias.

Storen also induced a grounder from pinch hitter Pablo Sandoval, but it rolled through the right side for an RBI single. Angel Pagan, the tying run, skied a blooper into shallow left.

The Nationals outfield had been playing ‘no doubles’ defense, stationed further back than usual. Zimmerman charged, called off Ian Desmond and made a sliding catch to end the inning. Zimmerman made ESPN’s “Baseball Tonight” on a routine basis at third base. For the first time, he cracked the highlights in left.

“The fantastic thing is, he has approached playing left field with vigor and with enthusiasm and has worked hard to be a really good left field,” Manager Matt Williams said. “That’s a compliment to him, and a compliment to what he thinks about our team.”

Zimmerman entered the experiment with modest expectations. The Nationals were confident Zimmerman possessed the athleticism to at least break even in left. So far, he has surpassed what anyone thought he could do.

“I knew I could catch the ball,” Zimmerman said. “People have always played outfield, messing around, their whole life. I’ve done that. I think it’s a small sample size to get any praise yet. I know I have a lot of work to do still, and I can get a lot better. . . . I feel comfortable. I’m starting to feel more comfortable than I felt a week ago.”

The Nationals had built their lead when Cain imploded at the start. Denard Span, Anthony Rendon and Werth drew consecutive walks to begin the game. Adam LaRoche rolled a two-run single up the middle, and before the Nationals had recorded an out, they led, 2-0.

Werth made a decisive dash home when Zimmerman tapped out to the right side of the mound, which sent the Nationals ahead by three. With two outs, Danny Espinosa drew another walk and Yusmeiro Petit started warming up in the Giants bullpen.

The Nationals couldn’t add to their lead, but Cain had walked four batters in the first inning. Nationals starters have walked four batters in June.

Roark perpetuated a dominating run of starting pitching. In the Nationals’ past eight games, their starters have struck out 54 batters and walked four. Over Roark’s past three starts, he’s allowed one run over 21 innings, yielding 17 hits and two walks with 19 strikeouts.

The Giants placed Roark in a tough spot just once, when Crawford’s RBI triple scored Buster Posey in the fourth. With the help of his defense, Roark extracted himself from further damage.

He struck out Brandon Hicks with a 94-mph two-seam fastball that hissed wide of the plate until the last second, when it tailed back over the outside of the plate. Roark has gained renown for his command, but anyone who doubts his pure stuff should watch that pitch.

Roark had done the hard part, and he needed only one more out to strand Crawford. Gregor Blanco ripped a groundball to the right side, and the crowd roared — a hit and a run, they thought. If a second baseman other Espinosa had been stationed at that position, it may have been. But Espinosa shuffled to his left and into the outfield, giving himself an additional few feet of range. He slid on his left leg, smothered the ball and rifled an accurate throw from his knees. The crowd moaned, and Roark trotted back to the dugout.

Werth tacked on to the Nationals’ lead in the fifth. Cain’s first pitch, a curveball, curled low and out of the strike zone, and Werth took an ugly, defensive hack. He fell into an 0-2 hole with another awkward swing at another curveball. He did not appear long for the at-bat, but then Werth has a knack for regrouping midstream. He took a ball, swatted three consecutive fouls and spit on another ball.

“He made me look bad,” Werth said. “I just hung on long enough to get a pitch.”

Cain hung a slider, and Werth smashed it two rows over the left field fence. The first homer of the series for either team — and Werth’s first homer since May 14 — sent the Nationals ahead, 4-1.

“He grinds at-bats,” Williams said. “He has the ability to just foul balls off. Even he’s if not seeing the ball well or he’s not feeling good up there, he’s got the ability to foul them off. He got a good pitch to hit.”

The Nationals extended their lead in the ninth, when Span and Rendon worked tough walks and loaded the bases for Werth. He bisected the diamond with a groundball single into center. Two runs scored, and out in the bullpen, closer Rafael Soriano sat down and ceded to Jerry Blevins.

“We could put the big, scary guy back in his cage and save him for tomorrow,” Werth said.

To maintain their claim as the hottest team in baseball, the Nationals only needed to protect the lead. The short-timer in left field, the once and perhaps future third baseman, did his part.

“People say — I’ve heard it 100 times in my career — ‘I’ll just go to the outfield,’ “ Werth said. “It’s not that easy. I’ve seen plenty that say, ‘I’ll just go to the outfield.’ A few weeks later, their career is over. For him to go out there, to really make the adjustment and to look as good as he does, it’s pretty cool. I’m happy for him. He’s having a good time with it. He’s making big plays, and he looks comfortable. It’s been a natural progression for him, and it’s been fun to watch.”