The Tigers’ Alex Avila celebrates his sixth inning grand slam home run with burly teammate Prince Fielder. (Gregory Shamus/Getty Images)

Stephen Strasburg looked over his left shoulder and watched the ball soar into the purple-black night sky. As he took two steps off the Comerica Park mound and turned, any hope that it may stay on his side of the fence disappeared. The crowd leapt up and down. The Detroit Tigers previously clogging the bases circled and celebrated together at home plate. Strasburg’s expression never changed as he stuck out his glove and asked the umpire for another ball. What other choice did he have?

The Washington Nationals’ season lurched closer to the precipice Tuesday night after Alex Avila’s grand slam in the sixth inning sent the Nationals to a 5-1 defeat. Strasburg had never in his career allowed a grand slam and Avila had never hit one. The simultaneous firsts cracked a tie game wide open, nullified Strasburg’s brilliance and led to the kind of loss the Nationals cannot afford, one with their ace on the mound and the score in their favor much of the night.

“I pitched well, minus one pitch,” Strasburg said.

If Avila’s blast had not exterminated the Nationals’ chances of catching the Atlanta Braves in the National League East, it had dealt them the most severe blow yet. Minutes before Avila belted the grand slam, Brian McCann clobbered a three-run homer to give the Braves an five-run lead in Atlanta. In a few moments, the Nationals’ had seen a decent chance at an eight-game deficit evaporate. By the end of the night, the Braves’ margin had swelled to a season-high 10 games.

“I really could care less what the Braves do,” shortstop Ian Desmond said. “We’re [three] games under .500, whatever it is. We got to worry about ourselves. That’s first and foremost. And then the rest will take care of itself.”

The Post Sports Live crew debates whether Mike Rizzo sending Drew Storen down to AAA Syracuse was the right move for the Nationals and the struggling reliever. (Post Sports Live)

Afterward, in a quiet clubhouse, Strasburg was asked if he could sense the Nationals’ time running short to make a run at the division.

“I pitch once every five days,” he said. “I give it everything I’ve got the day I pitch. And if it doesn’t turn out the way you hope, then you do your best to learn from it and get them the next time.”

After their 14-run outburst Sunday, the Nationals scored one or zero runs for the 29th time this season. The Nationals grabbed a lead in the first inning after Bryce Harper’s leadoff triple. They would not manage another run off Tigers starter Anibal Sanchez, who for years tormented them with the Marlins. Sanchez improved his career record to 9-1 with a 1.98 ERA over 21 career starts against the Nationals after his seven-inning performance Tuesday.

Strasburg cruised into the sixth inning having thrown only 59 pitches. He had lost the lead when No. 7 hitter Andy Dirks lined a double off the left field line in the fourth. But he had contained the Tigers’ fearsome, even overwhelming Miguel Cabrera, the undisputed Best Hitter On The Planet.

“I was thinking the way he was going, he probably would give me nine innings,” Manager Davey Johnson said.

And then, as Strasburg’s offense again yanked away any safety net, his start dissolved. Prince Fielder singled and Victor Martinez ripped a double down the line with one out. Strasburg fell behind Jhonny Peralta, 3-0, and eventually walked him to load the bases. Strasburg moved to another three-ball count against Dirks, and he somehow summoned the guts to throw a 3-2, bases-loaded curveball. Dirks swung and missed.

Up came Avila. Catcher Wilson Ramos called a fastball and stuck his mitt over the outside corner of the plate. Strasburg’s 96-mph heater missed the target considerably. It zipped over the inside third of the plate, into Avila’s power zone. He crushed the ball 10 rows into the right field seats.

“I mean, the way it’s been going, I felt like there wasn’t much room for error,” Strasburg said. “So I knew I had to go right after him and make him swing the bat.”

Strasburg found a measure of solace in his response the slam. He retired the next batter he faced, then steamrolled through a 1-2-3, two-strikeout seventh inning.

“That’s what we want to see in him,” Ramos said. “He never put his head down. He kept fighting.”

The concerns from early this spring about his handling of adversity had abated, but it did little to help the Nationals on Tuesday. In the first, Harper pulverized a line drive to right, missing a home run by a matter of inches in two directions. The right field wall at Comerica Park contains an indentation, like a medieval turret. Harper’s smash drilled the fence just to the right of the opening and just below the top of the wall.

It came off with such force that the carom bounded past right fielder Torii Hunter. Harper’s helmet could not keep up with him and only made it halfway to second. Harper slowed into third with a stand-up triple. Two batters later, Ryan Zimmerman bounced a single up the middle to give the Nationals a 1-0 lead.

Strasburg held the lead until the fourth, when Dirks’s double kicked up chalked and scored Fielder, who had walked. He would strike out seven Tigers, including Hunter looking at a curveball that had the veteran buckling and backpedaling out of the box. But his record would fall to 5-9, an indictment of his offense more than an indication of the way he has pitched.

The Nationals ended the night at 52-55. They would have to wake early Wednesday to face Justin Verlander and try to climb out of a hole that may already be too deep.