Clayton Kershaw and Zack Greinke -- one of best pitching duos ever?

There’s a reason why that recent four-game losing streak was the first one to befall the Dodgers in two years -- Clayton Kershaw and Zack Greinke.

They are the best pitching duo in baseball. And now in their third season together, it’s time for them to be considered one of the best in baseball history.

The Dodgers have what most consider the premier pitching duo of all time in Sandy Koufax and Don Drysdale. In the early 1960s, they won four of five Cy Young Awards back when only one award was issued for both leagues.

Kershaw has won three of the last four National League Cy Young Awards. Greinke won the American League honor in 2009 for the Royals and is the current favorite to win it this year in the National League.

But Kershaw appears determined to make a serious run at Greinke’s effort.

To this point, Greinke is 12-2, easily leads the NL in ERA (1.65), is first in opponent batting average (.193) and is tied for first with the Nationals’ Max Scherzer in WHIP (0.87).

After what was a slow start for Kershaw, he has taken an ERA that was 4.32 on May 21 down to 2.39 -- fourth-best in the NL. He leads all of baseball with 205 strikeouts. His opponent batting average is down to .202 (fifth), and his victory Wednesday over the Nationals raised his record to 10-6.

And all of Kershaw’s numbers are trending downward. In his last 14 starts, he’s gone 8-2 with a 1.30 earned-run average, 132 strikeouts and only 14 walks in 108 2/3 innings.

“I feel like every time they start you’re going to see something special,” said catcher Yasmani Grandal. “These two guys compete against each other -- and that’s a good thing.”

These are not just two talented pitchers, but two extremely competitive ones. Their work ethic and preparation is unquestionable.

In his first two seasons with the Dodgers, Greinke went 32-12 with a 2.68 ERA -- and was still overshadowed by Kershaw (37-12, 1.80 ERA).

It might have fueled Greinke’s competitive fire. After Kershaw flirted with a perfect game July 23 against the Mets in New York, Grandal recalled a conversation he’d had with Greinke after a spring game.

“Kershaw better watch out because I’m coming after him,” Grandal recalled Greinke telling him.

They’re pushing each other to rarefied air. They’ve sustained this excellence for three seasons now. And at the rate they’re going, by season’s end they may have cemented their names among baseball’s best pitching duos in history.