Manager Matt Williams takes the ball from Tanner Roark in the seventh inning of Monday night’s loss to the Orioles. (AP Photo/Nick Wass) (Nick Wass/AP)

As groundskeepers watered the warning track inside an empty Nationals Park on Thursday morning, Tanner Roark sat inside the first base dugout. One year ago to the day, he had made his major league debut, a moment tinged with improbability. In 2010, Washington had acquired him for Cristian Guzman. In 2012, he went 6-17 in the minors. In early 2013, Roark relieved for Class AAA Syracuse.

On Saturday night in Atlanta, Roark will take the mound with a 2.54 career ERA. The only starters with a lower ERA since Roark’s debut are Clayton Kershaw, Johnny Cueto, Zack Greinke and Jon Lester. This week, a writer from Sports Illustrated interviewed him. “Pretty surreal,” Roark said.

On the bench Thursday morning, Roark watched his Monday night start against the Baltimore Orioles on an iPad. He explained his thought process from first pitch to last the mental inner-workings of how he has transformed from a Class AAA rotation-filler to one of baseball’s most successful starters in one year.

First inning

Roark had not faced the Orioles. As leadoff hitter Nick Markakis came to bat, Roark stepped behind the mound and tugged on both pants legs.

The Post Sports Live crew debates what the odds are that the division-leading Baltimore Orioles and Washington Nationals meet in the World Series. (Post Sports Live/The Washington Post)

“I usually try to use my fastball as long as I can,” Roark said. “You don’t want to show everything too early. You don’t want to show all your pitches in the first. By the time you get to the seventh, they recognize it.”

Roark used two change-ups against Markakis, then induced a grounder with a change-up. Catcher Wilson Ramos “had a good idea that he would swing, and I had a good idea that he would swing, too,” Roark said.

Manny Machado stepped to the plate with 12 hits in his last 32 at-bats. “I had no idea,” Roark said. “I didn’t even know he was hot. He pulls the ball a lot. We were trying to stay away. The 2-0 pitch is up and in. I just didn’t have my greatest command.” Machado still hit a grounder to short.

Adam Jones lined out to left, and despite a quick inning, Roark sensed something was off. He vowed to stay aggressive.

“When [the count] gets 2-0, 3-1, I do not want to walk anybody,” Roark said. “I’d rather someone get a hit off me, hit a home run. I hate walks. Get 2-0, 3-1, all right. You know a fastball is coming, so here it comes. Be ready.

“Good pitchers can’t nibble and try to make perfect pitches every time. You got to make them want to swing.”

Second inning

Chris Davis came to the plate with one out. “Pound him inside. You don’t want him to get his hands extended — like that,” Roark said as he watched a fastball sail down the middle of the plate.

“When I’m in the game, I know when I miss sometimes,” Roark said. “But I don’t watch the ball. I don’t follow it out of my hands. So I don’t know where the ball ends up. I just look at the glove the entire time. That’s my focus point.

“You know how some pitchers, they go through their windup and they’ll look down? Or look off the side, how [Dan] Haren does? It doesn’t work for me. I need to focus on the glove the entire time.”

With two on and one out, Ryan Flaherty laced a first-pitch fastball to center field. Denard Span snagged it and fired to second for a double play. Despite another scoreless inning, Roark could tell he lacked his best stuff.

“For 33 starts you have in a year, you have your five no-brainers, everything is perfect,” Roark said. “And then you have your five total bad ones. And then you have your other 20 that are always going to be a battle. That’s what defines who you are as a pitcher, I feel: those meaty 20 starts you have to grind through.”

Third inning

In the second, Ramos smashed a solo home run and gave Roark a 1-0 lead. “It gives me confidence,” Roark said. “Like, okay, the boys are playing today.”

The lead disappeared. Roark fired a fastball over the inner half of the plate when Ramos set up outside. Caleb Joseph pounded it into the first row of seats.

“My two-seamer,” Roark said. “Mechanical-wise, it wasn’t there. You know it, but you can’t really think about it.”

As Joseph rounded the bases, Roark rubbed the new ball the umpire had tossed him. “It’s frustrating,” Roark said. “That hitter is over with. You can’t worry about it. Get the pitcher, get the next two guys.”

Roark worked around an error and escaped the inning when Jones popped a hanging slider to left.

“I got out of the inning, so I’m not worried about it,” Roark said. “Some pitchers, they’ll get really mad at themselves because that pitch wasn’t where they wanted it. But if you get an out out of it, learn from it, don’t be mad about it. Be happy that you got an out.”

Fourth inning

Roark had settled into his second trip through the lineup. “They’re thinking the same as the first at-bat,” Roark said. “The third at-bat, that’s when they grind out, and they know, ‘Okay, this is what he’s done to me twice.’ ”

Roark changed his approach with Nelson Cruz. “He’s got good reason why he’s got 29 home runs,” Roark said. “I threw him a first-pitch slider [in the second], and he swung at it. So maybe he’s looking for it next time.” Two fastballs led to an easy grounder.

Roark remained committed to pitching Davis inside. On 1-2, Roark threw his best pitch all night: a two-seam fastball that started inside and veered back over the corner. Davis knew it was strike three before the umpire called it. Roark had set up Davis with an 0-1 fastball way inside.

“I just started throwing my four-seam again a couple starts ago,” Roark said. “You throw four-seamers inside to lefties, and they know it’s going to be straight. You set them up for that two-seamer to run back across the middle. They just freeze.”

With two outs, J.J. Hardy took a 1-1 slider that broke out of the zone, then barely checked his swing on another. “Man, J.J.!” Roark said. “Those are two good sliders right there. I work out with him in the offseason, too. I’m going to have to have some words with him.”

Roark eventually threw Hardy a good 3-2 fastball over the outside corner, and Hardy roped it to the right field corner. “Just a good piece of hitting,” Roark said. “Oh, well. Get him next year. Or maybe in the World Series.”

Roark fired Flaherty a fastball. He fouled it away with a late half-swing. “It tells me he can’t get into inside pitches,” Roark said. “He leans out over the plate and still wants to pull it.”

Roark would make Flaherty roll over with a change-up. Later, Roark would think back to that pitch.

Fifth inning

Roark’s grind continued after the Nationals had made it 3-1. He went 0-2 on Joseph and needed eight pitches to retire him. “See that, right there?” Roark said. “I’m not even following through. I’m standing straight up.”

Still, the bases remained when empty when Markakis dug in for this third at-bat with two outs. Roark had retired him twice with groundouts on change-ups. Now he started him with an outside fastball for strike one.

“Maybe I’m thinking a little too much into it,” Roark said. “I’ve only thrown one curveball. I get ahead of him fastball and then threw a curveball. I threw it well, but I threw it right into his happy zone, down and in. He didn’t crush it, but he hit it well enough for it go out.”

Markakis’s homer soared into the Nationals’ bullpen, down the right field line, and made it 3-2. Roark recovered with a first-pitch slider to Machado, who lined out to left.

“This whole team, they want fastballs,” Roark said. “They want to put you on the ropes right away. I’m not giving into it or falling for it. When there’s free swingers and people that want to rip your fastball, I love to go inside on them. I don’t even care for a strike: You’re not swinging first pitch. That’s my mentality. It makes me mad, but I guess that’s a good thing, to be mad at the hitter. You got to have no mercy out there on the mound, you know?”

Sixth inning

“We’re still winning,” Roark said. “I’m still confident.” Mostly, Roark kept nodding along with Ramos. “I shake whenever I have a pitch in my head I want to throw, that the hitter is set up for,” Roark said. He retired Jones and Cruz with sliders, which brought Davis to the plate again.

Roark threw Davis a change-up on the outside corner, at the knees, and he swung to make it 2-2. Roark fired almost the same pitch, but this change-up started on the zone and faded away and into the ground. Davis swung and missed to end the inning.

“This one is at the knees for a strike,” Roark said. “This one’s going to be starting at the knees and then on the ground.”

Roark had thrown 84 pitches. With two outs and two on in the bottom of the inning, Manager Matt Williams let him hit.

“It’s my game to win,” Roark said. “I got the lead. I’m confident. I think about my previous starts, too. I’ve gone seven. I can get seven.”

Seventh inning

Roark walked to the mound with the belief his night would last another two innings. “You don’t want to throw all your chips in at that time,” Roark said. “If I went back out there for the eighth after a clean seventh, that’s when all my chips were going to be in.”

Roark had been throwing Hardy four-seam fastballs all game. He hooked an 0-1 fastball into the left field corner for a leadoff double. “That was an elevated two-seamer,” Roark said. “That’s not acceptable, any part, especially when it’s a 3-2 game.”

As Flaherty came to the plate, “I thought in my head, ‘change-up,’ ” Roark said. “And I settled with fastball.”

Flaherty leaned over the plate and drilled the pitch — not the one Roark wanted to throw — off the right-center field wall for a game-tying double.

“Eighty percent of the time, that pitch costs you,” Roark said. “You had something else in your head that you want, and you just shake yes to that pitch. You don’t execute it all. It’s just dumb on my part.

“Maybe I was thinking about [Hardy]. Maybe I was thinking, ‘He got a double off me.’ Maybe I was trying to be too perfect. It was not acceptable in any part. I shouldn’t have done any of that. It drives me nuts.”

The go-ahead run stood on second. Craig Stammen and Ross Detwiler warmed. “I didn’t even know,” Roark said. “I don’t ever look over there. I don’t ever look in the dugout. There’s no reason to. It’s me and the catcher.”

Joseph bunted Flaherty to third. With one out, the infield crept in. Pinch hitter Delmon Young walked to the plate.

“I knew I could get him on a slider,” Roark said. “I know what he wants to do. He’s a swinger. If I threw those ones that I threw to Hardy that he laid off, it’s either a rollover or a swing-and-a-miss. Then I have him set up to pound him inside, so he doesn’t get the ball out of the infield. He just gets a little jam shot to third or short.”

But Roark’s slider hung over the plate. Young whacked it into left field for a go-ahead single. Williams walked from the dugout to take the ball.

“Just mad,” Roark said. “Any time I get taken out of the game, I’m mad. I don’t care if I have 125 pitches. I just want to be in the game.”

The Nationals lost, 7-3. Roark watched the end of the game from inside the trainer’s room, receiving his typical post-start treatment. He dressed in street clothes, spoke to reporters and left the clubhouse.

“I focus on the next team,” Roark said. “Take that game, take that loss and use it as fuel to the fire against the Braves.”