The 27-year-old shied away from the pitch after he was victimized by a home run early in the 2010 season.
The date was May 4, and the Marlins were positioned to beat the Giants and Tim Lincecum at Sun Life Stadium. Carrying a one-run lead into the ninth inning, Nunez was taken deep by Aaron Rowand, who connected on a slider.
The Giants went on to win, 9-6, in 12 innings, and Nunez walked away from the game cautious about throwing the slider for the rest of the year.
An objective of Nunez in Spring Training is to re-establish the slider as his third pitch, to go along with an effective changeup and 96-mph fastball.
During the offseason in the Dominican Republic, Nunez worked on his slider. He threw at home with the Rays' Joel Peralta, who was once a teammate of Nunez when they both were with the Royals.
In his first two bullpen sessions at Marlins camp, Nunez has thrown a number of sliders. Thus far, the results have been good.
"He threw some good ones in his side session," pitching coach Randy St. Claire said. "Last year, we talked about it, too. He started to throw it, and he gave up a home run early in the year on a slider. A lot of people questioned it. It came out in the media. He got beat by the slider, his third pitch, and he basically shut it down."
After an up-and-down 2010, Nunez signed a one-year, $3.65 million contract, the highest one-year deal the Marlins have ever paid a reliever.
Nunez saved 30 of 38 chances a year ago, but he labored in the second half and lost the job to Clay Hensley.
If Nunez can add a reliable third pitch, St. Claire feels he could become a lock-down closer.
"That repertoire helps him become the guy he's capable of becoming," St. Claire said. "He is a guy, I think, who is capable of saving 45 games for you."
If Nunez falters, the Marlins will look at fallback plans. According to manager Edwin Rodriguez, Hensley would be the first option.
For now, the intention is to give Nunez every chance to feel comfortable closing out games.
Nunez said he is striving to keep the job for the entire year -- and in the years to come.
"I don't want to be [the closer] one year, then not be the closer, and then be it again," the right-hander said. "I have worked a lot on the slider in the Dominican. It has good movement. It's good. It's [getting] much better.
"In Spring Training, I'm working to throw it more in the game. [Hitters] now will have to look for a fastball, changeup and a slider. The more pitches, the better."
By shying away from the slider a year ago, it basically made Nunez a two-pitch reliever. Once teams became aware of that, hitters sat on either changeups or fastballs.
"He didn't want to get beat with that third pitch," St. Claire said. "Well, the only way he's going to get better is if he throws that third pitch. If he kept throwing it last year, he wouldn't be working on it this year. It would be down.
"It's a process. You're going to take your lumps sometimes on a pitch that you're just learning. But you just can't abandon it. If it's going to help you and the team in the long run, you just can't abandon it just because you got beaten by it one night."
Nunez still is relatively new to the closer's role. He moved into the position during the 2009 season, when he saved 26 games.
"You don't have to go out and throw 10 [sliders] in a game," St. Claire said. "All he has to do is let them know through the course of the year that he's going to get them out with a slider occasionally. Then it makes it so that when you get two strikes on a hitter, now a hitter can't sit on certain pitches."