Zack Greinke and Clayton Kershaw are competing with each other for the NL Cy Young, but together are becoming one of the greatest pitching duos ever.

Considering their dominant performances this year, Zack Greinke and Clayton Kershaw have sparked considerable debate as to which Dodger ace deserves the National League Cy Young Award. But beyond the accolade lies another compelling question: Where do Greinke and Kershaw rank among the great single-season pitching tandems in history?

One way to answer this question is by looking at Wins Above Replacement. The Fangraphs version of WAR focuses on the main factors over which pitchers have control, scales them against a common baseline level, and adjusts accordingly, depending on park factors and run environment. By setting aside team defense and accounting for other variables, the metric provides a solid starting point for comparing Greinke and Kershaw with their counterparts from different eras.

Below, we see the top 25 single-season pitching tandems since 1956 — when the Cy Young Award was established — through today. (A “tandem” is defined as the two starters with the highest WAR on the rotation.) Each tandem must be at a “superstar” level (i.e., have at least 5 WAR) to ensure a genuine partnership of aces, and the hurlers must also be teammates for the entire season.

Source: Fangraphs

As we can see, Greinke and Kershaw already crack the top 25 with 13 WAR, even though they’ve only made 58 starts and completed 408 2/3 innings. What’s more, Fangraphs estimates that they’ll have another eight starts and 48 innings before the season ends. If they generate an additional 1.3 WAR, as projected, their 14.3 total will boost their ranking to 13th.

And given the relatively conservative nature of the projection (e.g., Greinke and Kershaw have averaged 7 innings per start, not 6), it’s reasonable to say that they have a legitimate shot at the top 10.

Of course, a holistic metric like WAR can sometimes obscure important nuances in player performance. After adjusting for park and league factors, the pair also compares favorably to duos from other eras.

Specifically, Greinke and Kershaw have a combined 2.39 fielding independent pitching, which is 36 percentage points better than league average in 2015. This “64 FIP-” is sixth-best on the list. The mark lies in close proximity to the Diamondbacks’ Randy Johnson and Curt Schilling, who had consecutive FIP- marks below 60 in 2001 and 2002.

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As Dave Cameron has shown, there’s value in breaking down FIP and analyzing how well pitchers do in each of its major components: strikeouts, walks, and home runs (hit batters are also tracked, but they have low totals). In all three areas, Greinke and Kershaw rank among the top 10.

Most notably, their strikeout and home run rates are 45 and 39 percentage points better than league average — seventh best in both categories. The high strikeout rate is driven largely by Kershaw, of course, who has 264 Ks already, and is vying to be the first pitcher to reach 300 since Johnson and Schilling reached the milestone 13 years ago. Meanwhile, the low home run rate is attributable mainly to Greinke. A mere 7 percent of his fly balls have left the yard, placing him fourth among qualified starters this season.

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Overall, when we piece everything back together, it seems that Greinke and Kershaw are poised to claim their place among the ten best pitching duos over the past six decades. Even a more conventional analysis supports their case. With a combined 1.87 ERA, they outperform the league average by 50 percentage points, which puts them atop the list. Greinke alone is at a historically low 1.61, chasing the likes of Pedro Martinez, Bob Gibson, and Greg Maddux.

To be sure, this analysis is far from perfect. In recent years, with the advent of new technologies and methods, we’ve gained a better understanding of how FIP components themselves are often dependent on factors beyond the pitcher’s control. For instance, catcher framing and umpire bias can influence strikeouts. On the flip side, while hitters are predominantly in control of batted ball velocities, pitchers still have some measurable effect on them. These issues are far from cut and dry. If they make it difficult to isolate pitching from everything else, then we can only imagine how much more challenging it is to evaluate pairs of starters — let alone across time.

Nevertheless, given what we have, Greinke and Kershaw emerge in a more-than-positive light by historical standards. Their collective efforts should be trumpeted far more than the supposed Cy Young rivalry for which they’ve shown little concern.

(Contributed by Robert Tagorda.)