PORT ST. LUCIE, Fla. — Matt Harvey extended his index finger and traced a spiral in the air. For much of 2015, this was the movement of his slider, formerly his most devastating pitch and one that he essentially functioned without during most of his first season back from Tommy John surgery.

“It was just spinning,” Harvey said, demonstrating the slider that he featured for much of last year. “It wasn’t doing anything.”

By his own admission, not until near the end of the season did his slider resemble what it had been before surgery. But when Harvey took the mound Friday for his first session of live batting practice, his fastball rippled with the late life that had been missing. His slider looked as sharp as it did before he went under the knife in October 2013, just as it had down the stretch in 2015.

“I’m seeing stuff that he didn’t have last year,” Mets manager Terry Collins said after Harvey’s throwing session. “I’m seeing that late life, that last giddy-up. I’m seeing better spin on the slider.’’

Perhaps, as Collins suggested, the lack of sharpness on Harvey’s slider last season was merely the final vestige of the phenomenon he called “Tommy John hangover.” It’s not uncommon, after all, for pitchers to need a full season back to regain their form.

But now that Harvey’s slider is back, will he feature it the way he did in 2013? Is it worth the potential strain on his elbow?

“I just couldn’t really throw it last year until the end, so you never know how it’s going to come out,” Harvey said. “But today I threw a lot of good ones. That was kind of my pitch in 2013 and I kind of lived without it last year.”

Subscribe to Newsday’s sports newsletter Receive stories, photos and videos about your favorite New York teams plus national sports news and events. By clicking Sign up, you agree to our privacy policy.

One rival talent evaluator said: “I don’t expect he’s ever going to lean on that pitch as much as he did pre-surgery because 90-mph sliders put lots of stress on the elbow.”

Harvey’s use of the slider dipped from 18.5 percent in 2013 to 14.1 percent last season, though he said many of those came later in the season. But even without the slider, Harvey went 13-8 with a 2.71 ERA in 29 starts. By the end of the postseason, he had upped his career-high innings total to 216. The workload gives Harvey a firm base to build off last year’s successful comeback.

“We’ve talked about that Tommy John hangover; there is something to that,” Collins said. “This guy’s stuff is a little bit better this year and it was pretty good last year. So I’m very excited to see the way Matt’s thrown.”

According to the evaluator, by the time the Mets surged to the postseason, Harvey’s slider again had become “a true swing-and-miss pitch.”

“I don’t know if it was a forearm strength thing or a feel thing,” Harvey said. “But obviously, I got it back toward the end of the season.”

Harvey and the rest of the Mets pitchers will be eased into action in spring training. None is expected to pitch in the earliest of the team’s Grapefruit League games.

Instead, Harvey said his focus is moving at his own pace in preparation for what could be a special season.

“It’s always nice, a step in the right direction toward the regular season,” Harvey said after facing batters. “We’re moving along nicely. Everything feels great.”