Jose Altuve, right, and Carlos Correa celebrate the Astros’ 2-1 win over the Yankees in Game 1 of the ALCS in Houston. (Ronald Martinez/Getty Images)

By virtue of its having an odd number of innings and no clock to speak of, baseball does not lend itself, either mathematically or aesthetically, to division into halves the way other team sports do. But this month, subtly and organically, the game has cleaved itself into two distinct parts. With starting pitchers getting less leeway to go deep into games, and bullpens growing in importance and workload, postseason baseball has become a game of two halves: the starter half and the bullpen half. And only one of them lends itself to scoring.

No team understands the dichotomy as fully as the Houston Astros, and no opponent provides as stark a lesson in its significance as the New York Yankees, with their bullpen full of rocket arms. On Friday night, in Game 1 of the American League Championship Series, the Astros dominated the game's first half, as they have done all season, then ran out the clock, such as it were, for a 2-1 victory.

Game 2 is Saturday, with new Astros ace Justin Verlander facing the Yankees’ Luis Severino.

On a night when lefty Dallas Keuchel became the first Astros pitcher in this postseason to last beyond the sixth inning — delivering seven scoreless innings, striking out 10 and boosting his credentials as the premier Yankee-slayer of his era — the Astros took the lead with a pair of fourth-inning runs and made sure the Yankees’ diabolical bullpen never got a say in the outcome.

The Astros’ bullpen carried the slim lead home for the final two innings, with closer Ken Giles surviving a towering homer in the ninth by Yankees first baseman Greg Bird to earn a five-out save.

The Astros outscored opponents by at least 38 runs in each of the first four innings this season, accounting for 82 percent of their run differential of plus-196, and they have scored first in all five of their games this postseason. You don’t want to be behind when the parade of fire-breathing relievers begins, and these days it begins earlier than ever.

Entering Friday night’s game, relievers had accounted for more innings pitched (171⅔ ) than starters (165⅓ ) this postseason — a trend that, were it to continue through the end of the World Series, would mark a first for baseball’s playoffs, and perhaps a shift from which there will be no going back.

The Yankees, whose bullpen features five of the top 12 relievers in baseball by strikeout rate, present the biggest second-half challenge in baseball, expertly constructed for the purpose of extinguishing rallies and hope and locking down leads in the second half of games. But they can’t lock down a lead if they never get one, and the Astros, by winning the first half, made sure of it.

“That’s a team over there with some big arms, and they’re effective and they’re good,” Astros center fielder George Springer said of the Yankees’ relievers. “The goal is to have the lead and make them have to chase us, instead of us having to chase them with those guys on the mound.”

The Astros won the first half Friday night largely through the bat, glove and feet of second baseman Jose Altuve, who came to the plate each time to a chant of “M-V-P! M-V-P” and answered three times with singles, boosting his batting average this postseason to .579.

If you’ve wondered how a player listed at 5-foot-6 can be a leading candidate for most valuable player in a sport that doesn’t involve thoroughbreds or parallel bars, consider the way Altuve altered the course of the game in the fourth inning. First, in the top half, he made a diving play at second base to rob Didi Gregorius of a base hit.

Then, in the bottom half, he beat out an infield single, stole second on cannon-armed Yankees catcher Gary Sanchez and scored the game’s first run when Carlos Correa lined a hanging slider into left field for an RBI single. The Astros tacked on a second run when Yuli Gurriel singled home Correa two batters later.

“The hand-eye coordination is off the charts,” Yankees Manager Joe Girardi said of Altuve. “I mean, it’s really off the charts. He doesn’t swing and miss a lot. He’s got an unbelievable gift and he knows how to use it.”

It was the fifth time in five games this postseason the Astros scored first.

Friday night’s Game 1 felt something like a throwback to the old days — which is to say, 2014 or so — when aces regularly pitched deep into the second half of games, even in October. By the time the first reliever of the game, Yankees right-hander Chad Green, climbed the mound, it was the seventh-inning stretch and the crowd of 43,116 was beginning its nightly singalong to “Deep in the Heart of Texas.”

This series offers a delicious contrast of histories, atmospheres and even heights — as evidenced whenever the two leading candidates for MVP, Houston’s Altuve and New York’s 6-foot-7 slugger Aaron Judge, stand next to each other.

The Yankees, of course, have won 27 World Series titles, including five in the past quarter-century. The Astros, born as the Houston Colt .45’s in 1962, have won none and have only played in one, losing to the Chicago White Sox in 2005.

You could get a sense of the difference between the franchises as the players lined up, caps removed, along the baselines following the pregame introductions for the national anthem. Along the first base line, the Astros, in their Friday-alternate orange jerseys, sported mullets, mohawks, full beards and thick stubble. Along the third base line, the Yankees, in their classic road grays with no names on the back, were all buzz cuts and smooth chins.

And then Keuchel took the mound, with a beard that could house a couple of small farm animals.

Minute Maid Park’s retractable roof was mercifully closed, protecting the proceedings and the sellout crowd from the swelter residing just outside, where it was 86 degrees at first pitch. It also left the deafening roar, which went up with each big pitch.

While watering the field before the game, the grounds crew spent an inordinate amount of time and water on the area in front of home plate, leaving it a shade of dark chocolate that contrasted with the mochas and caramels of the surrounding dirt. This was by design — Keuchel is the most prolific groundball pitcher in baseball, and the muddy patch in front of the plate was there to slow down grounders off the Yankees’ bats.

But on Friday night, Keuchel’s chief weapon was the strikeout, with his 10 whiffs representing a season high. In his career, regular and postseason combined, he is now 6-2 with a 1.09 ERA against the Yankees, with 62 strikeouts and only eight walks in 57⅔ innings.

“It’s just a storied franchise,” Keuchel said of his dominance over the Yankees. “They have so much rich history, you almost don’t even need to get up for the game. You’re already up for it. That’s what they bring.”

Keuchel ran into his only real jam in the fifth, a first-and-second, nobody-out crisis, which he escaped with a couple of clutch pitches and one defensive gem. With two outs, Judge lined a single to left, but left fielder Marwin Gonzalez — a super-utility man who made at least 14 starts this season at five different positions — gathered it in and fired a strike home, measured at 97 mph by Statcast, to nail Bird, the slowest member of the Yankees’ lineup, at the plate.

Out beyond Gonzalez’s spot in left field, the Yankees’ relievers peered out of their bullpen through plexiglass windows. It was nearly halftime of Game 1, and they were waiting, armed and ready, as they would be all night, for a lead that never came.