The day before he took the mound Friday at Nationals Park for the home opener against the Atlanta Braves, Jordan Zimmermann couldn’t sleep. At around 2 a.m. Thursday, he felt sick to his stomach and vomited. He was suffering from what appeared to be the flu. The Washington Nationals right-hander was scheduled to start that day against the New York Mets but knew he wasn’t going to make it.

Instead, the fully rested Tanner Roark filled in against the Mets while Zimmermann flew home to Washington to rest and fill his body with fluids. Friday morning, he called Matt Williams to tell the manager he felt well enough to start, then allowed only one run and struck out nine over five innings in a 2-1 loss to Atlanta, a performance that saved his team’s bullpen from having to cover for him.

“Jordan pitched his heart out,” shortstop Ian Desmond said. “He did a good job especially given the circumstances. That’s what we expect from him. He’s a bulldog. Pretty impressed.”

Despite Zimmermann’s assurances, Williams wanted the training staff to evaluate him Friday. Zimmermann passed that test and only then did Williams allow him to start.

“Health-wise, I feel a lot better than I did,” Zimmermann said. “[Thursday] was rough. Long day.”

Zimmermann is unsure whether he had food poisoning or the flu, but several Nationals dealt with the flu and chest colds during spring training. Zimmermann said his stomach was still rumbling during Friday’s start but felt fine. Despite that, his expectations for his performance remained unchanged.

“I was going out there and expecting what I always expect out of myself and that was to go deep in the ballgame,” he said. “And when skipper wants to take the ball out of my hand that’s his choice. That was my mentality [Friday]. Go out there and go as deep as you can.”

Zimmermann didn’t show many signs of illness. He fired 95-mph fastballs to first baseman Freddie Freeman in a perfect first inning. Through the first four innings, he allowed only three base runners, including a hit batsman. He breezed through the lineup, striking out seven of the first 14 batters he faced.

“I was happy with the way the outing went,” Zimmermann said. “Everything was working. Slider was really good. I kept them off-balance for the most part.”

Zimmermann showed his first signs of struggle in the fifth inning. He left a 92-mph fastball up to Evan Gattis, who smashed it over the left-center field fence. Zimmermann then coughed up a single to Andrelton Simmons; catcher Jose Lobaton threw him out at second base. Zimmermann walked Jason Heyward with two outs and he stole second base, but Zimmermann battled B.J. Upton for six pitches and finally struck him out. His pitch count sat at 81 but that was enough for Williams.

“He didn’t eat much [Thursday] and very little [Friday] morning so didn’t really have a whole lot of energy going out there,” Williams said. “Last inning he started to labor a little bit. Ball started to get up in the strike zone. He was good though. For what he could give us today, he was good.”

If Zimmermann hadn’t been able to pitch, the Nationals would have used a combination of relievers, likely starting with Craig Stammen. But the aftereffects of a bullpen game could have hampered the relief corps for Saturday and even Sunday. This is why Zimmermann’s performance was big.

“We’re proud of Jordan going out there on pretty much zero energy, going out there and giving us five innings,” first baseman Adam LaRoche said.

james.wagner@washpost.com