Center fielder Rick Ankiel makes a game-ending diving catch off the bat of Jose Reyes on Wednesday night at Citi Field, giving Brad Peacock a victory in his first major league start. (Henny Ray Abrams/AP)

When Brad Peacock began this season at Class AA, he assumed he would not pitch in the majors this year, an attitude conditioned by his unconventional career path. Peacock played third base growing up and pitched only eight innings in high school. The Washington Nationals selected him in the 41st round of the 2006 draft. Expectation was a foreign thing.

The latest evidence of Peacock’s altered, elevated status arrived Wednesday night at Citi Field, when he made the first major league start of his career and shut out the New York Mets for five innings in the Nationals’ 2-0 victory before 26,885. The win vaulted the Nationals into a tie for third place in the National League East. It also validated Peacock’s ascent from unknown amateur to top prospect.

Against an only-in-September Mets lineup riddled with call-ups, Peacock allowed two hits and three walks in his five innings, striking out two over 94 pitches.

“I was really impressed with this kid tonight,” catcher Jesus Flores said. “He’s got great stuff. He could handle himself. His curveball was great tonight. It was really good to call the game for him tonight.”

Peacock, 23, attacked the Mets’ hitters in the same way he approached minor leaguers. He also carried another ritual with him.

After Peacock finished his warmup in the center field bullpen, he took the ball and flipped it to his father, Jerry, who followed him to every one of his minor league starts and saves the balls in a box.

“He’s got a lot,” Peacock said.

Peacock thought he would be nervous, but he was not. The first time he pitched in the majors, on Sept. 6, happened unlike how he imagined. He jogged in from the bullpen and faced Dodgers star Matt Kemp with two on.

“I got my feet wet a little bit,” Peacock said. “I knew what to expect.”

Last weekend, the Nationals named Peacock their minor league pitcher of the year, an honor he earned with his 15-3 record, 2.39 ERA and 177 strikeouts in 1462 / 3 innings across Class AA Harrisburg and Class AAA Syracuse. Peacock generated a buzz last fall with an eye-popping performance in the Arizona Fall League.

This summer, after improving the deception of his delivery with Harrisburg pitching coach Randy Tomlin, Peacock exploded. Baseball America ranked him the No. 42 prospect in baseball. At the end of the season, he was named the Eastern League pitcher of the year.

“It’s just great,” Peacock said. “You don’t want to stay down there in the minors. You work to get up here. That’s all I did all season, is work hard. It paid off.”

Peacock still carries traces of his unassuming past. He arrives at the park early, dresses in his batting practice uniform and then sits in the stool in front of his locker. He speaks softly and short in interviews, all earnest smiles and nervous laughter.

On the mound Wednesday night, any trace of timidity disappeared. He retired the first eight hitters he faced, a streak he broke when he walked Mets pitcher Mike Pelfrey. Peacock unveiled the arsenal that turned from a throwaway draft pick to a piece of the Nationals’ future. He fired his fastball between 92 and 95 mph and mixed in a vicious curveball.

“He’s got electrifying stuff,” said Randy Knorr, his manager at Class AAA Syracuse.

Said Flores: “He’s got a lot of movement on that ball. He throw a fastball that cuts and a sinker. Everything together with a curveball and the change-up, it worked out well.”

Peacock found his only trouble in the fourth inning. Ruben Tejada led off with a groundball single and moved to second on a wild pitch. Two walks loaded the bases with two outs. To the plate walked first baseman Josh Satin, whom Peacock had faced early in the season, when they both played in Class AA. He knew Satin was a fastball hitter.

Before the at-bat, Peacock circled to the back of the mound and took a deep breath. Peacock threw Satin a 2-2, 81-mph change-up, which darted down and inside toward the right-handed hitter. Satin popped up to third base, where Ryan Zimmerman settled under the ball and made an easy catch.

In his final inning, Peacock allowed a two-out single to Jose Reyes, but nothing else. Tejada flied out to right field on Peacock’s 94th pitch, a fitting end — 11 of Peacock’s 15 outs came on fly balls or popups.

“I usually get a lot of groundballs,” Peacock said. “That was actually surprising.”

The Nationals scored both of their runs in the third inning, on consecutive RBI singles by Michael Morse and Jayson Werth. The hits gave Peacock the lead. The first win of his career wasn’t sealed until the ninth, when Drew Storen earned a high-wire save only after he walked the first two hitters he faced and center fielder Rick Ankiel made a diving, game-saving catch in the left-center gap to rob Reyes. When Storen turned and watched the liner, he did not think Ankiel would catch it.

“I don’t think there’s really any camera angle, other than the one I had, that you can really describe how awesome that was,” Storen said. “That was one of the best catches I’ve ever seen. I was thinking, ‘Okay, hopefully he can stay in front of it.’ All of a sudden, the last 15 feet, he kind of kicked it into a whole other gear. I told Rick I owe him a couple dinners.”

The catch sealed Peacock’s win. When Peacock came out of the dugout to shake hands with teammates, General Manager Mike Rizzo gave him a hug. As he gave an interview, teammates smeared a whipped-cream pie in his face.

In front of his locker, clubhouse manager Mike Wallace handed Peacock three balls. Peacock asked Wallace not to write anything on them; Peacock’s mother likes to write on them. Peacock would flip one of those balls to Jerry, too, and he could expect there would be many more to come.