Washington’s Michael Morse, left, is congratulated by teammates Adam LaRoche, center, and Ryan Zimmerman after hitting a grand slam during the first inning in St. Louis on Saturday. (Jeff Roberson/Associated Press)

Never count your chickens until they’re hatched. But when an egg’s shell is covered with cracks and a beak is peeking out, that’s a clue.

On Saturday night, in a melodramatic, comic but almost heart-rending game, the Nationals broke through with a crucial 6-4 win in the 10th inning over the tough, hot Cardinals. If ever a victory felt like the precursor to a celebration, in a day or quite soon, this was it. Magic number: One. Heart palpitations from this game: countless.

Like most births, it was both scary and thrilling. The Nats took a 4-0 lead on Michael Morse’s first-inning grand slam. They blew that margin gradually, the last of the lead disappearing against Drew Storen in the bottom of the ninth as 42,264 red-clad partisans roared.

Then, finally, in the 10th inning, Kurt Suzuki, who once drove in the winning run of the College World Series for Cal State Fullerton, doubled home a pair of runs with a scorching line drive double off the left center-field fence. Sure, walk Danny Espinosa intentionally to get to Suzuki. “You want to make them pay,” Suzuki said. “These games definitely get you ready [for the playoffs]. I ain’t gonna lie. It’s tough games.”

If this one didn’t give you a hint of what the postseason feels like — keep that defibrillator handy — nothing could. Night after night of this in October makes you a baseball fan, if you live. But, exhausting as this win was, in its wake, the math and the momentum of the N.L. East race now live almost entirely on the Nats’ side.

“Do the numbers,” said Ryan Zimmerman of how hard a Braves comeback would now be. “This was a big win.”

With four games left in the season and a four-game lead in the National League East, the only way the Nationals don’t win their division is if they go 0-4, the Braves go 4-0 and the Braves then win a one-game playoff in Washington on Thursday.

Is that the “cheep, cheep, cheep” of a new Nats era that we hear being born?

In a packed house in new Busch Stadium, the Nats brought their “A” playoff game for the first six innings — and their best grit and good fortune to the final three — to win a battle between two contenders who could easily end up meeting in the postseason.

Morse hit a comic-opera of a grand slam — a faux run homer, if you will — in the first inning off Kyle Lohse (16-4) and Jordan Zimmermann looked like a man ready to assert himself in October with his fifth straight strong start.

First, let’s have the cheerful face of comedy before we clench our teeth.

If you want cosmic code that a team is rolling right, then the Morse blast in the first inning provided it. With a full moon rising behind the right-field stands, Morse bashed a line drive that barely cleared the fence, quickly bounced back on the field and, immediately, confused nine Cards fielders, four Nats runners and all four umpires.

Nats swarmed everywhere. Zimmerman, who started on second, somehow barely got past third base. A pileup ensued in his wake. Morse, trapped off first by 89 feet, but forbidden to advance by a surplus of Washingtonians in the vicinity, retreated to first base and was tagged out a foot shy of safety. Was the score 1-0 with two outs and men at second and third? Or were the Nats ahead 4-0?

Replay was conclusive, but Jeff Nelson’s crew, doing its homage to the departed fake NFL refs, added a twist few eyes have ever seen. With Nats everywhere (Bryce Harper, who’d scored, even came back from the dugout), the umps simply ordered everyone back where they’d been — including Morse to the batter’s box, sans lumber.

Morse took the prize for improv, doing a pantomime swing before starting ’round the bases again. “How am I supposed to get this started?” Morse said he thought. So, he faked a swing and even gave his trademark helmet slap. “If I hadn’t done it and they’d called me out, I would never sleep again,” Morse said.

Further proof of right living or just dumb luck was the Nats escape from one of Davey few managing gaffe of this (or any) season. But he almost pulled off a beauty.

At the end of baseball seasons in treacherous pennant races, it’s not always if you lose one game that matters, it’s how you lose. And if you do it badly enough, with enough eerie karma dripping from the hideous thing, it can incite further mischief.

When you have a 4-0 lead in the seventh inning with a fresh bullpen, life should be good. The Nats have done business one way all season with their starting pitchers. So why would they change now after 95 wins? When a starter gets past 100 pitches or gets in a jam past the sixth inning — or especially if both conditions exist — the manager picks the arm of his choice and waves it toward the bullpen.

In the seventh on this night, Zimmermann was cruising. That was the problem. There are unwritten rules. And one is that you don’t take out elite starters when they haven’t given up a single run. Suddenly, two rules of thumb were at war with each other. Zimmermann had thrown 104 pitches. And he’d just given up back-to-back clean one-out singles to Carlos Beltran and Skip Schumaker. All that says, “relief.”

“I never second-guess myself, but I’m thinkin’ about that one,” said Johnson, who, with hindsight might have called Burnett for the lefty Schumaker.

The bottom of the Cards batting order was up — No. 8 hitter Pete Kozma. What harm could he do? As it proved, plenty. Kozma lashed a double down the left-field line for two runs. Zimmermann stayed for five more pitches after that, walking ’11 World Series hero David Freese on his 112th pitch, his most of the year by four.

“I’m try to get Jordan ready to go deeper in games [as he matures],” Johnson said. “But, afterwards, I apologized to him. He said, ‘My bad.’”

Johnson was caught in a classic predicament, one that’s been a snare for other managers. Under the greatest pressure, they find it hard to stick with their season-long patterns and, instead, leave their most talented star on the mound when the game’s on the line, even if he is outside his normal comfort zone. Classic example: Grady Little and Pedro Martinez in the ’03 A.L.C.S. The manager says, “I’m going to die with my stud.”

And often they do.

When Grady and Pedro were cited to him, Johnson grinned and said, “Maybe the pressure’s gettin’ to me.” It’s not. But winning makes the stress easier to digest.

Experienced as the Cards are, they make mistakes, too. Storen got his blown save on a couple of seeing-eye ground balls hits and a sacrifice fly. But at least it wasn’t a gift. St. Louis reliever Fernando Salas committed the cardinal sin, walking leadoff man Adam LaRoche on four pitches to start the 10th. A fine Roger Bernadina sacrifice bunt led to Espinosa’s intentional walk. And that led straight to Suzuki.

“Cheep, cheep, cheep.”

For previous columns by Thomas Boswell, visit washingtonpost.com/