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Through the first three weeks of May, the Washington Nationals piled up wins, doing their best to file April’s deluge of defeats into a forgotten drawer. Losses like Saturday’s sloppy 8-1 stumble against the Philadelphia Phillies have not come so frequently, and can therefore be reasonably dismissed as bad days, rare reminders of mortality.

The Nationals fell behind early Saturday and could not come back, as Phillies ace Cole Hamels repelled a few weak charges. Hamels is still one of baseball’s best left-handed starters, and he can be especially tough when staked to a lead.

But defensive lapses — four errors’ worth — left Nationals Manager Matt Williams on the top step of the dugout, arms folded, dismayed. Another shaky performance by Stephen Strasburg — 3 2 /3 innings’ worth — sent the right-hander to the clubhouse early, head down, disconsolate. If there is cause for concern for a team that has won 18 of its last 23, that is probably it.

At times this year, as Strasburg battled injury and mechanical trouble, he did not pitch like a man who believed in his stuff. What the Nationals wanted, Williams said, was for Strasburg to pitch with conviction. If he is to be hit, he should give up hits against his best pitches — namely, his high-90s fastball — thrown with confidence.

Strasburg started strong Saturday but faltered again. He allowed the Phillies six runs, five earned, on seven hits and has failed to make it out of the fourth inning in three of his past four starts.

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“It’s definitely something that I’ve never experienced before. I think it’s a test,” Strasburg said. “It’s a test for me, and I’m going to look at it that way and I’m not going to quit. I’m going to keep going.”

Two of the blows that doomed Strasburg, a third-inning double by shortstop Cesar Hernandez and a two-run home run by third baseman Maikel Franco a few batters later, came on 0-2 pitches. Four of the seven hits he allowed came with two strikes.

“Today he was just outside the strike zone or outside the strike zone,” Williams said. “Just not throwing it where he wanted.”

Twenty of the 34 pitches Strasburg threw over the first two innings were fastballs. The Phillies got a hit on one of them. In the third inning, when the Phillies scored four runs on three hits in their second time through the order, Strasburg threw 13 fastballs of 22 pitches.

“He was aggressive early,” Williams said. “. . . then he got in a little bit of trouble and just couldn’t make a pitch when he needed to get out.”

In the fourth, three Nationals errors — Bryce Harper throwing home, Ian Desmond fielding a ground ball, and Wilson Ramos catching a throw home — aided a two-run Phillies rally. Strasburg threw seven fastballs on 15 pitches. Batteries mix things up more as the lineup turns over. Throwing the same fastball-heavy mix the first time through the order as the third time is often ineffective because hitters adjust. When Strasburg and Ramos mixed in the fourth, the Phillies hit a little of everything.

“I really don’t know,” Strasburg said. “I think I came out and I was pounding it good early and then I just stopped hitting my spots for some reason.”

Odubel Herrera doubled on a curveball. Hamels singled against a fastball, and Harper fired homeward, over cutoff man Ryan Zimmerman. Harper did not have a play at the plate, but Hamels moved to second as he tried to create one. Then Ben Revere singled on a change-up. Desmond’s error let Hernandez reach, and Williams pulled Strasburg. The pitcher trudged off the mound, an ace feeling like the joker once again, his stuff intact, but the results disappointing.

“It looks like he lacks a little bit of confidence out there,” Desmond said. “Maybe he’s fighting something mentally. But I have no doubt that he’ll shake it off and come back just as good as ever.”

Right-handed rookie A.J. Cole, a starter by trade but with the Nationals for situations like Saturday’s, relieved Strasburg. He allowed two runs on four hits in 4 1 /3 innings and struck out seven. He set down nine Phillies in a row at one point. The Braves shelled Cole in his major league debut in late April. In two relief appearances since, the player considered Washington’s most big league ready starting pitching prospect has allowed two runs and five hits in 7 1 /3 innings, striking out eight.

Right-handed reliever Casey Janssen, who missed the first seven weeks of the season with right shoulder tendinitis, made his Nationals debut in the ninth. The Nationals signed Janssen to be their setup man, a late-inning veteran to fill the void left by Tyler Clippard and others. Janssen is not a power pitcher, and his velocity sat between 86 and 88 mph. He hurried through a 1-2-3 ninth, his first inning as a National.

Meanwhile, the Nationals’ offense was quiet against Hamels. He shut them down through seven innings, allowing three hits and no runs until the eighth, when Desmond doubled home Michael A. Taylor, who had reached on an infield single. That was the extent of the day’s offense.

Like his team, Harper showed signs of mortality Saturday. He went 0 for 4 with two strikeouts and slammed his bat against the ground after the second one. Hamels finished eight innings having allowed that one run on five hits with five strikeouts.

The Nationals would have needed a powerful offensive showing to recover from the deficit Strasburg left them. His ERA is 6.50, and that in 44 1 /3 innings. He is 3-5 — and still striking out more than a batter an inning — but he is allowing more than 12 hits per nine innings. He allowed just more than eight last year, which was the most he had allowed for a season in his career.