Nationals third baseman Ryan Zimmerman makes another ill-timed throwing error in the fifth inning to open the floodgates for the Cubs, who plate four runs after the error — all with two outs. (Jonathan Newton/The Washington Post)

For the first four innings of Saturday’s 8-2 loss to the Chicago Cubs, Stephen Strasburg spun near perfection on the mound. His command looked impeccable, his fastball blazing and his offspeed pitches knee-buckling. He gave up only one hit.

Then, in one disastrous inning, the game went from a pitcher’s duel between Strasburg and Edwin Jackson to a deflating, shoulder-slumping end for the Washington Nationals. Ryan Zimmerman’s sixth error of the season with two outs in the fifth started the downward spiral, and Strasburg was unable to recover.

Following Zimmerman’s throwing error, a play that could have ended the inning, Strasburg walked two batters and allowed two singles, a double and four runs. Strasburg possesses a world of ability, but how he responds to adversity was exposed.

His performance began with brilliance and ended with a whimper. He was saddled with his fifth loss, though none of the four runs charged to him were earned.

“He’s too good a pitcher to let adversity behind him let him down,” Manager Davey Johnson said. “He’s certainly capable of picking us up. It’s a team effort. And errors are a part of the game.”

Strasburg breezed through the first inning, which has plagued him all year. Entering the game, seven of the 17 earned runs he had given up came in the first inning. On Saturday, the Cubs failed to get a runner on base until Anthony Rizzo’s two-out single to center in the fourth.

Then came the fifth-inning implosion. Strasburg got Nate Schierholtz to line out to center field for the first out. Luis Valbuena doubled into the right field corner. Roger Bernadina, filling in for Bryce Harper again, fired a bullet to cutoff man Danny Espinosa, whose relay to Zimmerman nabbed Valbuena going for a triple.

Two pitches later, Strasburg got Welington Castillo to hit a fastball to Zimmerman, whose throwing mechanics since offseason shoulder surgery have been much scrutinized. Though he had looked more comfortable and accurate throwing across the diamond since returning from the disabled list on May 3, his throw in the fifth pulled Adam LaRoche off the bag. Strasburg was already headed toward the dugout on the play.

“Just a bad throw,” Zimmerman said. “It’s frustrating. Stevie is throwing the ball well and has a heck of the game going. That obviously changed the momentum a little bit. I feel bad.”

Zimmerman again reiterated that his shoulder feels strong. His throwing errors have led to 11 unearned runs. The Nationals have been supportive of their cornerstone third baseman’s throwing as he regains his form, with Johnson again voicing his confidence on Saturday.

“It’s just working on coming back and getting back to where I was before all this stuff happened last year,” Zimmerman said. “You’ll feel good for a week or so at a time, and you’ll have a day where you do something like this. I wish I knew what to tell you. I wish I knew why it happens and why it doesn’t happen. Obviously haven’t been able to figure that out yet. It’s happening and have to keep working and keep going.

With Castillo on first base, Strasburg walked the No. 8 hitter Darwin Barney. He then coughed up a screaming two-run double off the right field wall to Jackson. Pitching coach Steve McCatty emerged to talk to Strasburg to no avail.

“I tried to tell him the other day after a situation like that, ‘You have to keep your head up every time,’ ” catcher Wilson Ramos said. “Never put your head down because if you put your head down you lose the focus. So you need to fight all game. That inning he pitched you have to fight.”

After McCatty’s visit, Strasburg walked David DeJesus, who then beat out a throw to second on a hard-hit single to Desmond by Starlin Castro. Then Rizzo singled to center to drive in two more runs. Strasburg trotted off the mound, not running behind the plate to back up on the hit.

“I may have a few words with him, but McCatty has had conversations with him about not letting anything bother you in the ballgame,” Johnson said. “And sometimes, he’s such a perfectionist, any little thing can . . . even when a guy flies out, he’ll wonder if he threw the pitch in the right spot. He’s very critical of himself. Tough one to let get away.”

Strasburg completed the inning and his outing at 95 pitches. He cruised through the first four innings with 53 pitches and seven strikeouts. In the fifth, Strasburg threw 42 pitches.

“I feel like I’m going out there and pitching well,” Strasburg said. “Just not happening on the days I pitch right now. It’s all going to change. It’s still early, and all I can do is just go out there and give everything I have every fifth day. Whatever happens happens.”

Left-handed long reliever Zach Duke, pitching on 11 days’ rest, allowed four runs and notched only two outs in the sixth, putting the game far out of reach. Zimmerman made his seventh error in the seventh, most for a third baseman, a fielding miscue that didn’t lead to any runs.

Meantime, the Nationals mustered two runs against old friend Jackson. Entering Saturday’s game, he carried a 0-5 record, 6.39 ERA and a four-year, $52 million contract, the type of money the Nationals weren’t willing to commit this winter for a fifth starter. Against the Nationals, Jackson allowed two runs over 51 / 3 innings.

The only runs were driven in by Desmond, who continued his torrid hitting streak. He finished 3 for 4 with a home run for the second time this series. LaRoche continued to emerge from his season-starting slump with another hit to extend his hitting streak to eight games.

The game, however, was determined long before the Nationals scored in the fifth and sixth innings. Zimmerman’s error and Strasburg’s inability to recover from it had sealed the Nationals’ fate.