Sean Doolittle’s focus is on improving his slider this spring. (John McDonnell/The Washington Post)

WEST PALM BEACH, Fla. — It took the Washington Nationals until four days before Opening Day last year to name a closer after evaluating their unproven options over six weeks. The uncertainty was an omen. By May, Washington was on its third choice.

There’s no angst this spring. Sean Doolittle is their closer, and, if there was any doubt, Manager Dave Martinez made it officially official Saturday afternoon, four days into spring training.

It’s the first time Doolittle has entered a major league season as the undisputed ninth-inning man. He inherited the role with the Oakland Athletics in 2014, when he amassed 22 saves in his only all-star campaign, and won the job in 2016 before a couple of bad weeks produced a quick demotion. But he excelled in the role when he joined the Nationals last July, compiling 21 saves in 22 chances with a 2.40 ERA.

“He’s got a closer mentality,” Martinez said. “I’ll tell you now he’s our closer. … He’s got a good fastball. He’s got a live arm and he likes to throw it. And for me, it’s letting those guys be who they are. He knows how to get guys out.”

But Doolittle is convinced there’s room for improvement beyond that fastball. And it starts with the Florida humidity.

This is the 31-year-old Doolittle’s first spring training in Florida after spending his previous springs in Arizona with the Athletics. Every spring he would try to find a feel for his slider and every spring it eluded him in the dry Arizona air. This year, he has noticed it’s better in his first two bullpen sessions of the spring. He attributes the disparity to the climate, and he hopes he can carry the success into the regular season.

“I’ve never known anything different,” Doolittle said. “Now being here I can see just like in my hand, the grip feels a little bit better. I can get it to do a little more coming out of my hand.”

Doolittle has always been a fastball-first pitcher. Only three qualified relievers — Kenley Jansen, Jake McGee and Zach Britton — have thrown a higher percentage of fastballs since he broke into the majors in 2012. His extraordinary ability to command the pitch within the strike zone has allowed for his success even when hitters know it’s coming.

But Doolittle has relied on the slider before. In that all-star 2014 season, 11.5 percent of his pitches were sliders. The ratio, however, dipped the next three seasons. There were days when the slider was crisp, but he usually didn’t have enough confidence in it to throw it. By the end of 2017, he was throwing almost all fastballs with a smidgen of change-ups.

“When I have an inning where I throw 12 fastballs out of 13 pitches, it’s not because I’m stubborn,” Doolittle said. “It’s because I don’t want to get beat on a pitch that I might not have as much confidence in. And I really want to change that.”

Doolittle believes adding more sliders to the mix will help avoid high pitch counts, one of the few faults that surfaced during his time with Washington last season. Relying on the fastball almost exclusively creates little margin for error. Even if a batter is surprised by a fastball’s location, Doolittle pointed out, he can recover in time to foul it off if that’s the pitch he’s expecting.

And when the location is off, the chances of a batter capitalizing are high. It happened in his only blown save with the Nationals last season, when the Pittsburgh Pirates’ Andrew McCutchen and Josh Bell ambushed him by jumping on first-pitch fastballs. McCutchen singled and Bell homered to tie the game in two pitches.

“There were times last year where, yeah, I put up a zero, I got a save, we won, but I threw 23, 25 pitches,” Doolittle said. “If I can do it in 12 or 13, over the course of a long season, that really could help out a lot. It’s still in the research and development phase, but the early results have been encouraging.”

Doolittle’s mentality isn’t changing. He still plans on attacking the zone with his mid-90s fastball when the games matter, and throwing too many sliders can lead to additional stress for someone who has been on the disabled list with shoulder injuries four times over the past three seasons. But his focus will be on his slider when the Grapefruit League schedule begins next week. He said having his role secured grants him the freedom to focus on the process of developing the slider this spring. He can throw it more often and in counts he wouldn’t otherwise without fearing poor results.

The objective is to have confidence in the pitch in the ninth inning on Opening Day. The Florida air is making it easier.

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