Go back to 2017-18, and check out the best relief pitchers in baseball across those two years as ranked by Wins Above Replacement.

Some of those names are still considered top relievers. Others are hurt, or struggling, or out of baseball entirely. But what's most interesting about the top dozen of that list is the veteran quintet of Big Closer Names who were at or near their peaks, and all somewhat similarly aged at the time, between 29 and 31 years old. Back then, you’d have looked at Craig Kimbrel, Sean Doolittle, Blake Treinen, Kenley Jansen and Aroldis Chapman, and perhaps thought they were the among the pinnacle of the art of relieving. Maybe they'd keep doing that indefinitely, even.

It didn't happen that cleanly, of course. It never does. It's the nature of relievers.

Kimbrel left Boston for Chicago and fell apart. Doolittle had a 4.26 ERA in 2019-20 as his fastball velocity collapsed. Treinen posted an ERA near 5.00 in 2018 and was non-tendered. Chapman has remained effective, unlike the others, but as he enters his mid-thirties, even he's had to adjust, since he no longer tops 100 mph with as much regularity as he once did. Jansen has been more "good" than "great" for the last few years, but he's off to a rough start in 2021, having struck out just a single one of the 16 batters he's faced, walking four.

It's not easy to maintain greatness, as other similarly-aged relievers like Dellin Betances (multiple injuries) are finding out, and we don't know yet if this is a bad start for Jansen or the beginning of the end. The other four, though -- Kimbrel, Doolittle, Chapman, and Treinen -- are all showing us something new in 2021. Here's what they're doing differently early on in 2021.

Craig Kimbrel, Cubs

What to know: His last regular season walk came last August
2021 to date: 14 batters faced, 9 strikeouts, 0 walk, 0 hit

Remember how this all went downhill for Kimbrel? It wasn’t a slow, graceful decline to middle relief work. It looked like a complete thud, the end to his career as a viable Major League pitcher at all.

After dominating for five years in Atlanta, one more in San Diego, and most of three in Boston, Kimbrel reached free agency. We do say most of three in Boston, because it’s important; late in that 2018 season, he began to have trouble finding the plate, and after walking eight in 10 2/3 postseason innings, it was Chris Sale who came out to finish off the Dodgers in Game 5 of the World Series.

With his fastball velocity also down a tick from previous years, the value of a ninth-inning-only reliever not what it once was, and the weight of a Qualifying Offer on his back, Kimbrel languished on the free-agent market until June 2019, eventually signing a three-year-deal with the Cubs. It ... did not go well. Kimbrel soon missed time with an elbow injury, and over 33 games as a Cub through the end of August 2020, he posted a 7.53 ERA. He allowed a 1.013 OPS. He was still missing bats, but he was walking entirely too many and giving up a homer every third inning.

On Aug. 1 of last year, Cubs GM Jed Hoyer noted “the shape of his pitches are not what they normally are,” but the bottom, the true nadir, came on Aug. 29 of last year, when Kimbrel entered the second game of a doubleheader in Cincinnati to finish off a 5-4 lead. Three walks (one intentional) and three wild pitches later, the Cubs had a 6-5 loss.

But then something fascinating happened. On Sept. 3, he struck out two Pirates without a walk. He did it again (2 K/0 BB) three days later, and three days after that, and two days after that, and before you knew it, Kimbrel had made it through September with 13 strikeouts and zero walks. (He did walk two in a postseason appearance against Miami. Let’s ignore that.)

That was enough for David Ross to declare Kimbrel would be the closer in 2021, and so far this year, he’s struck out 9 of the 14 hitters he’s faced, again without a single walk. Look, let's get nuts: Since Sept. 1, 2020, there have been 397 pitchers to face 35 hitters. Kimbrel has, without, hyperbole, been the most dominant of all of them.

So what happened? There's certainly something to be said for his first full Spring Training since 2018, and there's plenty of talk about improved mechanics. "Behind the scenes, Kimbrel got his delivery issues ironed out," wrote MLB.com's Jordan Bastian, "improving his direction to the plate and avoiding becoming too rotational."

It's probably all of those things. But part of it, too, has to be about how absolutely no one was going after his curveball when he would throw it on the first pitch last year, which points to either bad location or tipping it off or both:

This year, he's throwing it on the first pitch more than ever, a nearly 50/50 split with his fastball, and he's getting swings on it more than ever. Not at all coincidentally: Kimbrel is pitching from behind in the count less often than he has since he was a rookie in 2012.

Sean Doolittle, Reds

What to know: The once-great fastball might be back
2021 to date: 10 batters faced, 5 strikeouts, 1 walk, 1 hit

There was a time in the middle of 2018 when we actually called Doolittle "baseball's best reliever," and while that may have been a little too aggressive, it tells you a lot about just how effective he'd become after converting to the mound as an Oakland Minor Leaguer. Doolittle's approach was never mysterious, and it was never complicated: four-seam fastballs. High in the zone. Lots of them. Between 2014-18, Doolittle threw 86.2% four-seamers, the highest mark in baseball, and he was throwing them high before it was cool to do so.

Doolittle was fantastic for six years with the A's before being traded to the Nationals in 2017, and then he was incredible in '18, striking out 10 times as many hitters (60) as he walked (6) and posting a 1.60 ERA. He wasn't quite as good in 2019, though still effective, and it was easy to pin any struggles on a sore right knee and the heavy workload forced upon him by the early season disaster that was the Washington bullpen.

Turns out, it was just the beginning of a larger problem, because in 2020, Doolittle was downright bad, posting a 5.87 ERA with career worsts in both strikeout rate and walk rate. It's true that he wasn't healthy -- another injured list stint for the knee, and another one for an oblique strain -- but worse, the trademark fastball had lost its life, and velocity. (In 2018, remember, he'd had the most rising action of any fastball in the game.)

You'll see in this annual velocity chart here that 2021 looks a lot better, but focus on that dip in 2020. It's enormous.

Having turned 34 in September, there was clearly a ton of concern about what Doolittle's future would hold, which is why he was able to score only a modest one-year, $1.5 million deal from the Reds, and so far, so good. The velocity is better. The fastball rise is back at the top of the leaderboards. He's started incorporating a new breaking ball, one he's calling a curve.

So what happened? It's easy to say "he's healthy," and there's probably truth to that, but it also sounds like he gained a great deal of benefit from working out at Push Performance in Arizona this winter, where several Reds coaches already worked. "He started working with weighted balls," wrote Cincinnati.com, and the breaking ball work was "his first dive into pitch design with the slow-motion cameras and pitch-tracking devices."

It's a story we've heard a lot over the last few years. Is it enough to make Doolittle look like he once did? Cincinnati is quickly becoming known as a place where pitchers go to get better. Doolittle's journey might have began before he even got to Ohio.

Aroldis Chapman, Yankees

What to know: He's fully incorporating secondary pitches
2021 to date: 7 batters faced, 6 strikeouts, 1 walk

No typos there. Chapman has faced seven batters so far this season. Zero have put the ball in play. One, Baltimore's Pedro Severino, managed to draw a walk, but only after getting to a full count.

Chapman, obviously, occupies a slightly different zone than the other three pitchers here, because he never had a period of on-field struggle in the same way -- though he did miss most of the first half of last season due to a COVID-19 diagnosis. Still, he was as dominant as ever, better even, posting a 49% strikeout rate that was his highest since 2014. Still, Chapman is 33 years old, and his fastball was a career-low 97.2 mph last year. Is it absurd to see "97.2" and worry at all about a decline in velocity? Of course it is. But does that number stand out that much these days? Not ... really.

To that end, near the end of the year, we saw Chapman incorporate a new pitch, a splitter. He threw it six times, and the first three ended with strikeouts. (“As the years go by, you develop more as a pitcher," he told Lindsey Adler of the Athletic this spring.) When he threw it during a Spring Training game this year, it coaxed quite the reaction from an on-air Aaron Boone:

It was, though the fact that it's something else the hitter has to think about to go along with high-90s heat is half the point. Flash-forward to this year, and Chapman has thrown it five times, collecting four whiffs. Here it is getting Cedric Mullins; here it is getting Trey Mancini. Because he's Chapman, he's throwing it at 89.9 MPH, or about as fast as Corey Kluber's fastball, just with 10 inches of extra drop. It is deeply unfair.

What's more, because he's throwing his slider nearly a third of the time these days, he's actually now down to only about 50% fastballs, a far cry from his early days when that would be nearly 90%.

Oh, and the four-seamer? The all-time great historic heater that got him to this point in the first place? It's back up to 99.2 mph. No wonder he's allowing literally no contact.

Blake Treinen, Dodgers

What to know: Check out that new slider
2021 to date: 10 batters faced, 3 strikeouts, 2 walks, 2 hits

Treinen had his moments for Washington, but it wasn't until he was traded to the A's in 2017 -- in the deal that sent Doolittle back, ironically -- that his career really took off. In 2018, he picked up Cy Young votes as Oakland's closer, striking out 100 in 80 1/3 innings while allowing a mere seven earned runs, for a 0.78 ERA. ("Sometimes," he told MLB.com about his success that year, "it's more valuable to strike somebody out," as compared to an earlier emphasis on ground balls. The next year, that ERA ballooned to 4.91 as he had difficulty throwing strikes, and Oakland let him go after the season.

He signed with the Dodgers for 2020, and he was solid if unspectacular in the regular season before collecting some big outs in the run to the title. They brought him back on a two-year deal last winter, and, as first noted by Daniel Brim, so far this season, his slider has been something completely different. It's not being thrown harder, or with more spin. But it is moving more, a lot more. More than four times as much.

It is, for the moment, the slider with the most horizontal break in baseball, since Chaz Roe is expected to miss months with a shoulder injury.

Now, Treinen has only thrown it five times, though he did get swings and misses on it twice. We're not sure what this is yet, if anything. But if there's one potential weakness on the otherwise heavily favored Dodgers, it's at the back of the bullpen, where Jansen has gotten off to a considerably less-than-dominant start. But if it is something, well, that something looks like this. Good luck hitting that.