You'll never guess who's MLB's best reliever

The Brewers have the most dominant reliever in baseball. That doesn't sound like news. You're probably thinking, "Yeah, of course, Josh Hader is great."

Well, we're not talking about Hader. We're talking about Devin Williams. Hader hasn't allowed a hit this season ... but Williams has actually been the more lights-out reliever.

It's not because Williams has the lower ERA (0.64 to 1.69, thanks to Hader's five-walk blown save against the Pirates on Saturday), although that is a fun fact given Hader's hitless season. It's because Williams is striking out more hitters -- he has 29 K's in 14 innings, more than two per inning -- walking way fewer and suppressing opposing hitters' quality of contact better.

For the Brewers, it doesn't really matter who's the most dominant between Hader and Williams, because they have both, and it's the best relief tandem in baseball. But you already know all about Hader. It's time to take a closer look at Milwaukee's 25-year-old sophomore setup man.

Here, check out Williams' numbers.

No one is striking out more hitters than Williams

Strikeouts are the best thing a pitcher can do. Williams has been the best pitcher at doing the best thing a pitcher can do.

Highest strikeout rate, 2020
Of 320 pitchers with 50+ batters faced
1) Devin Williams (MIL): 52.7% (55 BF, 29 K)
2) James Karinchak (CLE): 50.0%
3) Edwin Díaz (NYM): 46.9%
4) Josh Staumont (KC): 43.1%
5) Shane Bieber (CLE): 42.4%
Hader: 39.5% strikeout rate (43 BF, 17 K)

If he pitched a full season with a strikeout rate that high, Williams would set a Major League record. Aroldis Chapman (52.5% in 2014) and Craig Kimbrel (50.5% in 2012) are the only pitchers with a strikeout rate of 50% or higher in a season. (Hader has two seasons in the top 10 -- 47.8% in 2019 and 46.7% in '18.)

Williams has gotten opponents to whiff on 50.5% of their swings against him, giving him the highest whiff rate in the Majors out of the 287 pitchers who've generated at least 100 swings. Hitters have taken 111 swings against Williams; they've only made contact on 55 of them.

Almost 40% of the pitches Williams has thrown this season have gotten either a called or swinging strike -- the third-highest rate in MLB. Only James Karinchak and Jake Diekman -- two of the other best relievers in baseball this year -- are ahead of him. A high called + swinging strike rate indicates that Williams doesn't just have nasty stuff, but that he's also commanding it.

That also shows up in the stat K-BB%. Williams' strikeout rate (52.7%) is 41.8 percentage points higher than his walk rate (10.9%). That's the best differential of any pitcher (ahead of just-traded former teammate David Phelps), and that's another leaderboard you want to be on top of. Hader, by contrast, only has a 10-point difference between his K% and BB%, because of his extraordinarily high 25% walk rate.

On top of Williams being the hardest pitcher to put a ball in play against in the first place, even when hitters do make contact, it's poor contact. That separates him from pitchers like Edwin Díaz, who has top-tier strikeout and whiff rates but gets shelled when hitters put the bat on the ball.

Not Williams. His 83.0 mph average exit velocity allowed is among the lowest in the league, and so is his 26.3% hard-hit rate allowed. Because of that, Williams' trio of Statcast expected metrics -- expected batting average, expected slugging percentage and expected ERA -- are all elite.

The xBA against Williams based on his quality of contact allowed is just .107. The xSLG against him is just .211. And his xERA is just 1.64. All three of those marks are in the top 1% of MLB. Williams ranks fourth in expected ERA of 381 pitchers who qualify for the league leaderboard.

Lowest xERA, 2020
Based on quality of contact allowed, plus K and BB
1) Caleb Ferguson (LAD): 1.33
2) James Karinchak (CLE): 1.42
3) Paul Fry (BAL): 1.62
4) Devin Williams (MIL): 1.64
5) A.J. Cole (TOR): 1.73
Hader: 3.22 xERA

Williams is really, really good. And it's also no secret why he's really, really good.

His changeup might be the filthiest pitch in baseball

Williams' changeup is unhittable. Literally. He hasn't given up a hit on it all year -- in 31 at-bats.

Hitters are 0-for-31 with 21 strikeouts in plate appearances decided on Williams' changeup. They're whiffing on nearly three of five swings they take against it, and when he throws it with two strikes, he strikes out the batter nearly half the time.

Hader, with his four-seamer, and Williams, with his changeup, are the only two pitchers this season to throw at least 100 of any one pitch type without allowing a single hit on that pitch. But Williams' changeup has determined the outcome of way more plate appearances.

Lowest BA allowed on a single pitch type, 2020
Minimum 30 PA decided on that pitch type (411 pitcher/pitch type combos)
1) Devin Williams' changeup: .000 (0-for-31)
2) Cristian Javier's slider: .059 (2-for-34)
3) Diego Castillo's slider: .061 (2-for-33)
4) Aaron Nola's 4-seamer: .067 (2-for-30)
5) Tyler Glasnow's curveball: .087 (4-for-46)
Hader: .000 BA vs. both 4-seamer (0-for-13) and slider (0-for-19)

It's a unique pitch. The way Williams pronates his wrist as he delivers his changeup gives it spin almost like a screwball. Teammate Brent Suter has been trying to figure out how to throw it for weeks now, only to find Williams' changeup basically impossible to replicate.

When you watch how Williams throws that thing, you start to understand why hitting it has been a fruitless effort. The movement on Williams' changeup is insane. He gets the best horizontal movement of any changeup in baseball and the fourth-best vertical movement. In other words: it drops and it fades at an elite level.

Changeups with most horizontal movement above avg., 2020
Among qualifiers with 50+ changeups thrown
1) Devin Williams (MIL): +4.8 inches
2) Tony Watson (SF): +4.7 inches
3) Chris Bassitt (OAK): +4.4 inches
4) David Peterson (NYM): +3.6 inches
5-T) Mike Minor (OAK): +3.5 inches
5-T) Miguel Castro (NYM): +3.5 inches
5-T) Felix Pe?a (LAA): +3.5 inches

Changeups with most vertical movement above avg., 2020
Among qualifiers with 50+ changeups thrown
1) Noe Ramirez (LAA): +8.9 inches
2) Trevor Cahill (SF): +8.2 inches
3) Devin Williams (MIL): +7.8 inches
4) Jake Arrieta (PHI): +7.5 inches
5) Logan Webb (SF): +7.3 inches

Williams' changeup drops an average of 40.2 inches and breaks horizontally an average of 18.1 inches. Both of those rank as the fourth-most overall changeup movement for anyone who regularly throws the pitch.

Normally, when you think of a changeup, you think of a pitch that "tumbles." Because of how pitchers grip a changeup, to make it slower and give it its typical movement, it's a low-spin pitch type. The average MLB changeup has a spin rate of 1,772 rpm.

But Williams' changeup just doesn't follow the rules. His extreme style of release gives him a high-spin changeup. He's averaging a spin rate of 2,827 rpm, which would be very high for a curveball ... and is, incredibly, over 1,000 rpm higher than a Major League average changeup. Williams' changeup spin rate comes in at 400 rpm higher than anyone else's (John Means is the closest to him at 2,444 rpm), an enormous gap.

The spin Williams puts on his changeup is crazy for that pitch type. But when you think about his release, and see his changeup moving like a lefty curve, it all starts to make sense.

And when you look at how that high-movement, high-spin, un-replicable changeup pairs with Williams' high-90s four-seam fastball, it starts to make sense why no one can hit him.

Which pitch is coming is a coin flip -- Williams is throwing his changeup 49% of the time and his fastball 45% of the time (plus a few cutters sprinkled in). He can, and does, throw either offering whether he's ahead or behind in the count; he can double or triple up on the change without fear, because it's so nasty, or he can sequence it with his fastball, because they tunnel perfectly.

The same way Hader's rising fastball is his signature pitch, but he can couple it with his slider to devastating effect, Williams' freakish changeup is his signature, but he can couple it with his fastball to be equally devastating. Actually, right now? More devastating.