Jayson Werth catches a Wil Nieves fly ball during the Nationals’ win over the Phillies on Thursday. (John McDonnell/The Washington Post)

An unsettling notion loomed over the first portion of the Washington Nationals’ season, a familiar feeling they needed to ward off. They spent two months trying to find themselves, never healthy, occasionally brilliant, frequently agonizing. As days peeled off the calendar and losses mounted, they began to wonder whether the same forces that ruined last year, when they waited too long to play their best and high expectations fizzled, had returned this year.

“We were digging deep there for a while and searching,” Nationals reliever Tyler Clippard said. “We had to take a look in the mirror. We all know what happened last year. We’re very aware of that and didn’t want it to happen again this year.”

In the past week, culminating with Thursday’s sweep-sealing, 4-2 victory over the beleaguered Philadelphia Phillies at Nationals Park, the Nationals provided the most resounding evidence yet that 2014 will not be like 2013. Doug Fister fired seven stellar innings in the latest suffocating start from a National, helped by several of his own did-you-see-that defensive plays. Adam LaRoche socked a two-run homer off Phillies starter Kyle Kendrick, the latest blast in a team-wide offensive resurgence.

The Nationals won five of six and swamped the Texas Rangers and Phillies by a combined score of 38-12 — no National League team has scored more or allowed fewer runs in the past week. It may have happened against the most injury-riddled team in the majors and one of the worst teams in the NL. But the Nationals splattered opponents for two series in a row, welcomed Ryan Zimmerman back and, earlier than last season, played up to expectations.

This is who we are,” said Clippard, who tossed a scoreless eighth. “It’s very uplifting. We’re coming into every game a lot more confident. Now that things are going like we know they can, we’re playing like we’re capable of. We’re seeing the results we want to see. It’s very encouraging. The energy is good. The vibes are good. And that’s important.”

Nationals beat writer Adam Kilgore talks about whether Ryan Zimmerman will eventually return to third base when he rejoins the team. (Post Sports Live/The Washington Post)

A greater challenge awaits on the West Coast, where the Nationals will play three games against the San Diego Padres, then move on to San Francisco and St. Louis for seven games against NL standard bearers. The Nationals departed with more confidence than they’ve had all year.

“You got to start somewhere,” LaRoche said. “We can build off this. You’re seeing more guys comfortable and confident at the plate, taking some walks and driving runs in.”

Zimmerman’s return deepened the lineup and lifted the entire roster. “When he came back, the team took on a different air of competition,” said closer Rafael Soriano, who notched his 12th save.

But the Nationals’ recent streak, during which only Yu Darvish stopped them, started four days before Zimmerman came off the disabled list. Fister credited togetherness, which he said helped the Nationals thrive the day after a 1-hour 48-minute rain delay. LaRoche said the offense had finally taken pressure off the pitchers. Manager Matt Williams credited improved defense and an offensive outburst from leadoff hitter Denard Span, who has batted .444 over the six-game tear. LaRoche said the Nationals’ offensive awakening has allowed starters to settle in.

There was no question Thursday — Fister took control. The Nationals have won each of his past five starts, a span in which Fister owns a 2.23 ERA while walking two and striking out 26. On Thursday, he allowed four hits and struck out five, using only 93 pitches before Williams lifted him for a pinch hitter.

Three batters into the game, Chase Utley’s RBI single up the middle gave the Phillies a 1-0 lead. Fister retired 17 of the next 19 hitters, allowing only one of them to advance past first base. When Jimmy Rollins doubled with two outs in the third, Fister responded by striking out Utley swinging at a 74-mph curveball.

“He’s awesome, isn’t he?” said right fielder Jayson Werth, who gave the Nationals their first lead with an RBI single up the middle. “You got to love playing behind a guy like. He’s into the game. He works fast. He throws strikes.”

Fister made an arresting impression with acrobatic defense. In the first, after Ryan Howard’s grounder to LaRoche, Fister raced to cover first as the Nationals attempted to complete a 3-6-1 double play. A first baseman in junior college, Fister did a full split, both legs flat on the ground, as he lunged to catch Danny Espinosa’s throw. That the ball trickled out of his glove made the play no less striking.

“I asked him if he was all right,” LaRoche said. “I thought he blew out. He hopped up and was like, ‘No, I’m good.’ Like nothing happened. I couldn’t do it.”

Said Fister: “It kind of reverts back to being a first baseman in college. It’s part of being a pitcher. You got to get over there and cover. It’s just something that comes naturally to me. Flexibility, I guess.”

In the third inning, Ben Revere slapped a grounder back up the middle. Fister, who drifts to the left side of the mound after he finishes his delivery, fell to the ground, stopped the ball with the reflexes of a hockey goalie, spun on his right leg, hopped to his feet and tossed the ball to LaRoche for an out. In the sixth, Fister vacuumed Revere’s leadoff bunt attempt and flipped to first.

With the Phillies in their wake Thursday night, the Nationals boarded a flight to San Diego. The next 10 games will help define their summer. At least they know they can play as they are capable.

“We’ve still got a lot to prove to ourselves,” Clippard said. “Going into this West Coast swing, playing against some teams that have been playing well and we need to beat, if everything goes well, we’ll hit the ground running.”