Luke Gregerson and Ken Giles slide their way to dominance

In the 2014 offseason, when the Astros made it clear that they were sick of the cellar, they set out to acquire some relief pitching. After Andrew Miller snubbed them, they picked up a suitable Plan B in Luke Gregerson. The reliever had a reputation as a ground baller who could pick up some Ks, and he didn't stray from that in his Houston debut. His 2.82 DRA and 78 cFIP over 61.0 innings helped the team make the playoffs for the first time in a decade.

Apparently, that didn't satisfy the Astros. Once the season came to a close, they swung a costly deal for the Phillies' Ken Giles. The high-octane hurler's 2.87 DRA and 77 cFIP in 2015 seemed to make him the perfect complement to Gregerson. The pair of righties would lock down the late innings — in some order — and ensure that the team returned to October. Or that's what Houston hoped.

Four-and-a-half months into the season, how has the duo performed? Here's a fun leaderboard:

1. Luke Gregerson — 20.1 percent
2. Ken Giles — 19.1 percent

Those are the major-league leaders in whiff rate among pitchers with 40 innings or more. Both Gregerson and Giles fooled hitters before 2016 — they entered the season with respective whiff rates of 14.5 and 15.2 percent — but this is unprecedented for each of them. It turns out that, for once, the Astros' hopes have come true.

What's made these relievers more deceptive? I'll answer that question with another interesting leaderboard:

1. Ken Giles — 34.1 percent
2. Luke Gregerson — 33.0 percent

These are the major-league leaders in whiff rate — on sliders. Among the 148 pitchers with at least 200 sliders thrown this year, no one has driven it past hitters more frequently than these two bullpen arms. This dominant offspeed pitch has blown away everyone to step in against these 'Stros relievers.

The progress hasn't come from anything else, either. Both Gregerson and Giles are essentially two-pitch pitchers; the former supplements his slider with a sinker, the latter with a four-seam fastball. This season, as their heaters have stagnated, their sliders have taken off:

Now, Gregerson and Giles aren't mirror images. They've each emphasized different aspects of their sliders as the reason for their success. Giles focused on the slider's interaction with his four-seamer: In March, he told ESPN's Jayson Stark that the offspeed pitch has "given two worries to a hitter," who can't catch up with it because he "[doesn't] know which one [he's] going to throw." That certainly matches up with the data — in addition to having a large fastball-slider velocity gap, Giles uses his straight slider to play off his rising heater:

Gregerson, on the other hand, cares more about movement. The following month, in an interview with FanGraphs' David Laurila, Gregerson said his ideal slider would offer "late, hard movement, not a change of speed from [his] fastball." And while Gregerson's arsenal can't match the velocity diversity of Giles, he more than makes up for that with movement. His slider sets itself apart by running down and away:

To recap: Gregerson throws a slow sinker and slower slider, neither of which rises much; the slider, however, has more horizontal movement. Giles throws a hard four-seamer and (relative to that) a much slower slider; while the slider's pretty straight, it contrasts with the vertical movement on the heater. So each of these pitchers has thrown an exquisite slider, but the similarities end there, right?

Not exactly. This season, both pitchers have turned up the heat on their sliders in the same manner — by burying them down:

Most pitchers won't elevate the slider — they generally want hitters to swing over the top of it — but this drop in location does stand out. Per Baseball Savant, Giles has thrown 67.8 percent of his sliders in sections 13 and 14 of the strike zone (here's a quick reminder of where those are); Gregerson has hit those areas on 63.4 percent of his sliders. Those marks rank sixth and 20th, respectively, in the major leagues. Here, their paths intersect, and for the better.

Interestingly, neither pitcher has improved much overall. Giles has a 2.61 DRA and 76 cFIP this season; Gregerson's DRA is 2.63, while his cFIP is 72. A sole swing-and-miss pitch, even one as elite as these sliders, won't make a shutdown reliever. That's especially true if his walk rate rises (as Gregerson's has) or if he doesn't get as many grounders (as Giles hasn't).

Still, the fact that they've perfected their sliders counts for something. Gregerson and Giles are already great pitchers, as the Astros expected them to be. Their monstrous offspeed pitches have put them on the path to historic greatness. If they manage to perfect their harder offerings as well, the Rangers — and the rest of the American League — will have some serious problems down the stretch.

. . .

All statistics as of Tuesday, August 17th.

Ryan Romano is a contributing editor for Beyond the Box Score. He also writes about the Orioles on Camden Depot and MASN Sports, and about the Brewers on BP Milwaukee.

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