Through the first four innings of his most recent start, Clayton Kershaw’s seasonal numbers said he wasn’t quite right. The Dodgers ace’s stats were still elite compared with most pitchers’: 2.56 ERA, 26.7 percent strikeout rate, 3.9 percent walk rate. But Kershaw has established such an extreme standard — only Walter Johnson has thrown as many innings before turning 30 while so suppressing runs — that we’ve grown accustomed to comparing him to himself.

And by that standard, the lefty was lacking, posting his highest ERA since 2010, lowest strikeout rate since 2013, and highest walk rate since 2015. He’d also been hit harder, yielding seven home runs in his first seven starts and allowing his highest average exit speed and rate of “barrels” per batted ball in the Statcast era. Coupled with lingering anxiety about his durability in the wake of the back injury that cost him two months last summer, Kershaw’s slowish start seemed to threaten his historic streak of lowering his career ERA in eight consecutive seasons.

Then came the last three innings of his start on Friday against the first-place Rockies at Coors Field. In those innings, Kershaw faced the minimum nine batters, allowing one single but erasing the runner on a double play. In his final frame, the seventh, he struck out the first two batters and retired the third on a dribbler just in front of home plate. Normally, a three-inning sample wouldn’t mean much. In this case, though, the encouragement came not just from the outs, but from the way Kershaw recorded them. While finishing off the Rockies, Kershaw showed signs of regaining the bite on baseball’s best breaking pitch, whose relative toothlessness had reduced Kershaw to merely being one of the nastiest pitchers in the game instead of the undisputed no. 1.

To hear his then-teammates tell it, Kershaw’s slider debuted in a bullpen session in May 2009. The pitch, which appeared fully formed, completed his formerly predictable repertoire, enabling him to take a large leap between his rookie and sophomore seasons. The more he threw it, the better he got: Last year, he threw it a third of the time and recorded his best rate stats ever.

From 2014–16, Kershaw’s slider was his most effective pitch. FanGraphs’ pitch-type run values assessed the slider’s worth at 2.6 runs per 100 pitches, besting all other qualified starters who threw a slider regularly in those years. This season, though, the slider’s results had dropped off, resulting in negative value for the first time in any of Kershaw’s full seasons. Entering his most recent start, Kershaw had allowed a batting average of .281 on his slider after keeping it below .200 in every season since 2009.

He’d also recorded his lowest whiffs-per-swing rate with the slider in the same eight-plus-season period.

Results are subject to some fluctuation over a seven-start stretch, but the slider’s attributes, too, seemed to have changed. After hovering between 88 and 88.5 mph in each of the previous three years, Kershaw’s slider had jumped up to nearly 90 this year.

At the same time, the pitch started staying up, dropping 2 inches less, on average, on its way to the plate.

The best predictors of a slider’s whiff rate are its vertical movement and its separation in speed from the fastball — two of the factors that determine how likely it is to deceive the hitter who’s looking for something straight. The greater the vertical movement — that is, the less drop — the fewer swings-and-misses a slider tends to induce. Likewise, the smaller the separation in speed between the slider and the pitcher’s fastest pitch, the less bat-missing ability the slider tends to have. Since Kershaw’s slider was sinking less and cruising closer to his fastball’s speed than ever before, it made perfect sense that some of his strikeouts were missing.

As Dodgers manager Dave Roberts acknowledged earlier this month, the slider’s erosion actually started last season, after Kershaw’s return from his herniated disc. The pitch’s speed and vertical movement both spiked in September and October 2016 before climbing further this spring, consistent with the theory that Kershaw was overthrowing, or failing to “finish,” the pitch. “The late depth, the late bite or life to the slider [just] wasn’t there, uncharacteristic,” Roberts said after Kershaw allowed four runs and two homers to the Giants in a six-inning start on May 1. “There were some sliders that were elevated.”

Kershaw continued to struggle with the slider early on last Friday, this time missing down as often as up. Look at his last three sliders of the fourth inning: low, lower, and lowest.

Those pitches ranged in speed from 90.5 mph to 90.9 mph, and in vertical movement from 6.9 inches to 7.2 inches. Kershaw doesn’t discuss his strategy in detail with reporters, but maybe the lousiness of those last three sliders, none of which came close to being a strike, made him determined to do something different.

The first slider he threw when he came out for the fifth traveled 85.3 mph, more than four miles per hour slower than the slowest slider he’d thrown to that point in the outing. The next one went only 83.7 mph, making it the slowest slider he’d thrown since June of last season — before he went on the DL. Contrast those three lousy sliders from the fourth with these four slower, tighter offerings from the seventh.

None of those pitches topped 88.7 mph. All of them sank more than the fourth-inning duds but were located higher and much closer to the center of the strike zone. If we zoom in on Kershaw’s sliders in that single start, the change in velocity and vertical movement in the last three innings becomes clear (with the caveat that MLB’s pitch-tracking technology has been a little less accurate this season than it has in the past).

If we average the values of the 12 sliders Kershaw threw in the fifth, sixth, and seventh innings last Friday, we find that they were much closer to his pre-injury sliders in speed, vertical movement and location, and called-strike probability than the ones he threw between his return from the DL last year and the end of the fourth inning last Friday (postseason sliders included).

Of course, a stretch of 12 more successful sliders doesn’t necessarily signal that Kershaw’s breaking-ball issues are behind him for good. (“I still don’t think he has his best slider,” Dodgers third baseman Justin Turner said after Friday’s start.) He might lose his feel for the pitch in his next start on Wednesday as quickly as he seemed to regain it last week. What we can say for sure, though, is that Kershaw belongs on the short list of baseball’s best pitchers even when his filthiest pitch is malfunctioning. When it’s working, no one can match him — and if his most recent sliders are any indication, the pitch could be rounding into form for the first time in months.

All slider stats courtesy of park-corrected data provided by Pitch Info.