Nationals starting pitcher Jordan Zimmermann leaves the game after giving up back-to-back home runs to the Cubs. (Charles Rex Arbogast/Associated Press)

His facial expression betrays nothing, but Jordan Zimmermann’s actions late Thursday afternoon told the surprising truth: He was, for maybe the first time since he pulled on a Washington Nationals uniform, rattled.

Zimmermann’s dazzling performance had just come undone when the Chicago Cubs’ Aramis Ramirez hit a two-run homer to break a 1-1 tie in the seventh inning at Wrigley Field. His pitch count had reached 105, but Zimmermann stayed in. Carlos Pena stepped to the plate, and Zimmermann, with the bases empty, threw from the stretch.

“Yeah,” Zimmermann said afterward. “I might have just, uh, I really don’t know why. That homer may have still been in my head.”

Pena completed the ugly end to Zimmermann’s brilliant outing, mashing a home run that factored into the Nationals’ 4-3 loss to the Cubs more than anyone could have known at the time. When Rick Ankiel’s flyball died at the warning track for the final out, the run the Nationals had produced after loading the bases with no outs in the ninth inning left them one shy of tying the game.

For 100 pitches on a sun-splashed day at Wrigley Field, Zimmermann had been brilliant, firing his fastball past hitters, fooling them with his slider, gaining strength as the game wore on. But Zimmermann had never before allowed a home run after his 100th pitch and remained on the mound, and when he did Thursday he learned a tough lesson.

“You’ve got to buckle down a little more, and when you get in a tight spot, you’ve really got to focus and make a pitch,” Zimmermann said. “I thought I focused. I just didn’t make the pitch.”

The Nationals dropped consecutive games to the Cubs, ruining their chance to win a road series for the first time under Manager Davey Johnson, who is 16-23 since taking over. The Nationals’ offense, continuing a season-long trend, only enhanced Zimmermann’s degree of difficulty. In 22 games started by Zimmermann this season, the Nationals have averaged 3.3 runs.

The offense’s most disappointing moment came in the ninth inning. The Nationals loaded the bases with no outs against Cubs closer Carlos Marmol, a single by Jayson Werth sandwiched between walks by Jonny Gomes and Laynce Nix.

Marmol’s pitches dart and dive like balloons losing air, and his brilliance is matched by his wildness. On Wednesday night he struck out all three Nationals he faced, but “it depends what day you get him,” Werth said.

By the time Marmol started Ian Desmond with two balls, it was clear the Nationals were getting him on an erratic day. Desmond, though, did not alter his initial approach.

“With a 2-0 count, he can hit a double,” Johnson said. “I’m not going to put a take on.”

Marmol fired a slider that broke to edge of the strike zone. Desmond swung, fouling the ball into the catcher’s mitt. With one pitch, the tenor of the inning had changed.

“Once you get to 2-0, I’m assuming he’s going to throw a fastball,” Desmond said. “He throws a slider, and he gets back in the count. He tricked me.”

Marmol fed Desmond four more sliders, and he fouled three away before whiffed at the last one in the dirt for the first out. Wilson Ramos scored a run by chopping the ball off the plate for an infield single. Brian Bixler struck out, leaving the game to Ankiel.

“I’m looking for a pitch to drive,” Ankiel said of the situation against Marmol. “He can be nowhere around the zone, and then he can throw three straight base-hit, should-tie-the-game” pitches.

Ankiel took two balls, fouled off the next pitch and then found the kind of pitch he wanted. He lofted a slider over the plate to right-center. The 34,733 home fans held their breath before it settled into Marlon Byrd’s glove.

“I thought it had a chance,” Ankiel said. “I just got under it a little bit.”

The final rally would been enough if not for the seventh, when Zimmermann took the mound having thrown 88 pitches. He struck out pinch-hitter Tony Campana swinging at a 93-mph fastball. He jumped ahead of leadoff hitter Starlin Castro, 0-2, and rifled a 94-mph fastball underneath his chin. Zimmermann then snapped a biting, low-and-outside slider. Castro flailed. He could not have hit it with an oar.

So Zimmermann was operating at the height of his powers when Reed Johnson walked to the plate. He had struck out seven while allowing six hits and two walks. His pitch count had climbed into the high 90s, so this would be his final inning, perhaps his final batter.

Johnson, though, has been a pest all series, having smacked four hits Wednesday night. Zimmermann fired an 0-2 fastball, and Johnson kept the inning alive with a single, his third hit. Up came Ramirez. Zimmermann threw him a 1-2 slider, low and away, and Ramirez lunged over the plate, hooking it to left field.

The line drive would have been a double at most parks, maybe an out. At Wrigley, it snuck into the first row of seats. Johnson thrust his fist in the air when he saw the ball sneak over the fence. Zimmermann wiped his brow.

Pitching coach Steve McCatty trudged to the mound, and left with Zimmermann still there. Zimmermann nodded when Wilson Ramos called for a change-up – a pitch he had not thrown all day. “Probably the worst choice I made all day,” Zimmermann said. “I don’t really know why I threw it.”

Pena launched the pitch down the right field line, ending of the few starts Zimmermann has left before he reaches his 160-inning limit, which the Nationals will uphold purely out of caution. “If I didn’t know any history about him,” Johnson said, “it wouldn’t be in my thought process.”

The two-year anniversary of Zimmermann’s ligament-replacement surgery will arrive Aug. 19, in one week. The effects of the operation have faded, a fact Zimmermann again made emphatically clear Thursday afternoon. It just was not good enough for the Nationals to win.