The surprising reason behind Zack Greinke's return to dominance

Rockies, Diamondbacks impressing in NL West (1:28)

Tim Kurkjian explains why the Diamondbacks and Rockies have been so successful this season. (1:28)

A year ago, I was at Arizona Diamondbacks camp late in spring training. The day I was there, Zack Greinke, the team's new $206.5 million acquisition, threw a bullpen session. Tony La Russa and Dave Stewart stood behind Greinke and watched him throw his first dozen pitches or so. That was all they needed to see. Greinke looked fine, methodically working through his repertoire, and the two executives moved on to something else. It had been a great spring, optimism was high, and everyone couldn't wait for the season to start.

Then A.J. Pollock got injured, the season began, and everything fell apart. Greinke would go 13-7, but he finished with a disappointing 4.37 ERA while missing some time on the disabled list. Stewart was fired, manager Chip Hale was fired, and La Russa got booted to "chief baseball analyst" as the team finished 69-93.

Maybe that optimism seen in 2016 was warranted. Maybe the team La Russa and Stewart built actually wasn't half-bad. The Diamondbacks head into the weekend with a 31-19 record and the second-largest run differential in the majors behind only the Dodgers. The key: The pitching staff, last in the majors in ERA in 2016, ranks fourth, with the rotation, led by Greinke, second only to the Cardinals with a 3.48 ERA.

Greinke is 6-2 with a 2.82 ERA and is second in the National League with 78 strikeouts, one behind the Giants' Jeff Samardzija. His strikeout rate, which fell off to 20.1 percent last year, is up to a career-high 29.9 percent. That's the sign Greinke is back. Heck, he's even throwing pitches like this 65 mph beauty:

That pitch, as fun as it is, isn't the primary reason for Greinke's improvement. He's throwing his slider more than ever, 26.2 percent of his pitches, compared to 20.6 percent last season. (More sliders are coming, with fewer changeups.) The thing is, the slider was pretty effective last year as well:

The swing-and-miss rate is a little higher this year, but the overall production allowed on plate appearances ending in sliders is similar. Buried in the numbers, however, is this: Greinke's called-strike rate on sliders in 2016 was 14.3 percent; this year, it has jumped to 21.8 percent.

Baseball is complex, with so many intermingling factors. Is Greinke getting more called strikes on the slider because of better location, which would then lead him to throwing the pitch more often, or is he getting more called strikes because the Diamondbacks jettisoned Welington Castillo as their primary catcher?

On a team level, the called-strike rate has gone from 29.9 percent to 30.9 percent. What does that 1 percent mean? Is this the result of better pitch framing from Jeff Mathis and Chris Iannetta? (They share the catching duties with holdover Chris Herrmann in a three-way job share.) Better pitching? Probably both, although Castillo has a reputation as a poor pitch-framer, ranking at minus-9.4 framing runs last year, according to Baseball Prospectus. (Iannetta actually ranked worse, but he rates as positive this year.) Mathis was signed explicitly for his reputation of framing pitches. Oh, and an important note: Mathis has caught all 10 of Greinke's starts.

Anyway, 1 percent may not seem like much -- one pitch per hundred, maybe two per game. But on sliders and curveballs, there's a noticeable difference. From year to year, the called-strike percentage on those pitches has gone from 25.4 to 28.3.

That's a notable difference. If the Diamondbacks are getting a few extra called strikes a game because of their catchers (mostly Mathis), that's significant. A few extra called strikes have the ripple effect of setting up other pitches as well, and it can give the confidence to throw that pitch again. That's not the entire reason the staff ERA is much lower, but it's certainly a part of it.

In fact, it's possible Greinke isn't really all that different from last season. He had four blow-up starts last season in which he allowed seven or more runs, and those games ballooned his ERA. His strand rate last year was 71.9 percent, which ranked 61st among 84 pitchers with at least 100 innings. This year, he ranks sixth among qualified starters with an 86.3 percentage. He's leaving fewer runners stranded, he's throwing more sliders, and he has Jeff Mathis behind the plate. Add it up, and the D-backs have their ace.