Joy abounds in the dugout after Anthony Rendon scores in the first inning to give Washington all the offense it would need Saturday at Nationals Park. (Jonathan Newton/The Washington Post)

Before he trotted to the Nationals Park mound for the ninth time Saturday afternoon, Tanner Roark stood next to Sandy Leon in the Washington Nationals’ dugout. The crowd roared in anticipation, and Leon felt nervous, because he had never before caught a shutout. Before they headed back to the field, Roark turned to Leon. “One-two-three, let’s go,” Roark told Leon. “Have fun with it.”

“He wasn’t nervous at all,” Leon said later.

The major leagues have yet to find anyway to frazzle Roark. Nobody else, himself included, can find an explanation for the staggering emergence he continued Saturday afternoon. Roark led the Nationals to a 4-0 victory over the San Diego Padres with a three-hit shutout, the team’s first complete game of the season. Roark retired the first 16 batters he faced, walked one, struck out eight and needed only 105 pitches for the first complete game and shutout of his brief, stunning big league career.

“It’s definitely humbling to come out here and compete like I know I can compete at this level,” Roark said. “It feels really good. A game like today keeps building your confidence even more.”

Roark, 27, entered spring training as a candidate, not a lock, for the Nationals’ rotation. He leads Nationals starters with a 2.76 ERA, and has pitched more innings (322 / 3) than anyone on the staff but Stephen Strasburg (34) — though Strasburg has made six starts to Roark’s five. Once a competitor for a spot, Roark has now entrenched himself on the roster.

“When he takes the mound, he feels like he’s under control to all of us,” Manager Matt Williams said. “There’s certainly a trust factor there, that he is prepared and he is able to go about it and throw strikes.”

Roark announced himself last year as a call-up in August, going 7-1 with a 1.51 ERA. So many factors pointed to Roark being exposed as a fluke: He was 26; he had gone 6-17 in 2012, one of six minor league seasons; he had been traded in 2010 in a deal for Cristian Guzman; he had been left exposed in the Rule 5 draft after 2012 – any team could have acquired him for $50,000.

And yet, with every start, Roark further establishes himself as a bona fide big leaguer. In 35 career innings at Nationals Park, he has allowed one earned run. Roark has made 10 major league starts. In five, he has not allowed an earned run.

“You do it enough times, and it goes past being a fluke or a question mark,” first baseman Adam LaRoche said. “He’s fun to play behind. He works quick, throws strikes, and he’s not afraid of contact. That’s kind of his thing. He’s a bulldog out there.”

The Nationals’ awakening offense pounded 10 hits and staked Roark to a three-run lead in the first inning against Andrew Cashner, a hard-throwing right-hander who entered with a 2.12 ERA. Ian Desmond went 3 for 4 and socked an RBI double into the right-field corner, one of five hits in his past eight at-bats.

Roark required nothing more. The Padres never moved a runner past second base, and after eight innings he had thrown just 90 pitches. Roark finished his start by striking out Jedd Gyorko, San Diego’s cleanup hitter, with a 92-mph, high-and-outside fastball. He punched his glove, pointed at Leon and, for the first time all day, cracked a smile.

Roark hugged Leon, shook hands with teammates and stood by the Nationals’ dugout for a television interview. Relievers Drew Storen and Tyler Clippard dumped two Gatorade coolers on him. “Real cold,” Roark said.

With the team having played 19 games in 19 days — and No. 20 on tap Sunday — the Nationals’ bullpen had grown weary. They had a day off scheduled for Monday, and aside from precautionary warmup tosses thrown by Tyler Clippard in the eighth and Rafael Soriano in the ninth, Roark gave the Nationals’ relievers a bonus break. “Beyond mammoth,” Storen said.

“It’s easy for me,” Leon said. “Because you can call whatever, and you know he’s going to throw it for a strike. You don’t have to think too much.”

Roark’s game plan could be abridged to one word: attack. He threw first-pitch strikes to the first seven hitters he faced and 23 of 31 in total. “Strike one is the best pitch you can throw,” Roark said. “So that’s what I try to do.”

After five innings, Roark had allowed one ball out of the infield and no base runners. He knew what might be possible – “every pitcher knows,” he said – but he told himself to keep throwing strikes, nothing more.

Roark’s streak snapped when catcher Rene Rivera flared a soft line drive into center. Roark allowed only two more singles, one by Gyorko in the seventh and one in the ninth by Chris Denorfia. After the only walk he issued, with one out in the eighth, Roark induced a double play on the very next pitch.

Roark ditched his straight, four-seam fastball and started throwing darting two-seamers in 2013, after he briefly moved to the bullpen at Class AAA Syracuse. Roark credits a mental makeover for his sudden success. The two-seamer provides the best physical explanation. He can throw at a left-hander’s belt buckle, and it turns into a strike.

“It’s huge,” Roark said. “Greg Maddux used it a lot. You see guys jumping out because they think it’s going to hit them, and then it cuts across the plate. It’s a very effective pitch.”

Between his previous start, when he shut out the Angels for 62 / 3 innings, and Saturday, Roark discussed the pitch with former Nationals pitcher Livan Hernandez, who now works as a coach. Hernandez lived on the sinker, and in Roark he sees another pitcher capable of lasting.

“Nobody can hit that pitch if he throws it in the right spot,” Hernandez said. “And the curveball is amazing. If he pitches like that, he’ll be in this game forever.”