PHOENIX — When Carson Kelly stepped to the plate in the ninth inning, there was a palpable tension.

Seconds earlier, it looked as if the Arizona Diamondbacks were set to grab their first walk-off win of the season.

A perfect throw from the reigning American League MVP Mookie Betts to gun down Eduardo Escobar at home plate following a single from Nick Ahmed extended the game.

Kelly ended it.

The catcher, making his first home start as a D-back, roped a two-out, walk-off single down the left field line to give the D-backs a 5-4 win over the defending World Series champion Boston Red Sox.

The win secured a series victory over Boston and pushed Arizona (5-4) over .500 for the first time this season.

“You’re like ‘oh, so close.’ Maybe we’ve got to hit it a little bit left. Nick did a great job and had a great at-bat there and it sucks because you’re so close,” Kelly said. “I was like ‘you know what, let’s hit it down the line or something.’

“I’m just happy I could come through for our team today.”

Kelly’s hit was a fitting ending to a game that showcased the strengths and weaknesses of the D-backs so far.

Starter Luke Weaver improved on his D-backs debut against the Dodgers — he allowed five runs in 4.1 innings against L.A. — by allowing just three runs in five innings.

All three runs came in an ugly second inning.

Xander Bogaerts walked to start the inning and scored two batters later on an Eduardo Nunez groundout.

It went from bad to worse when Red Sox starter David Price singled in a run to push the lead to 2-0. It was just the pitcher’s fourth hit in his 12-year MLB career and his first RBI.

An Andrew Benintendi ground-rule double pushed the Red Sox lead to 3-0, a seemingly daunting deficit against the former Cy Young winner.

How’d the D-backs respond? They scored four runs of their own in the bottom of the second inning.

A team that was expected to see an offensive down tick following the losses of Paul Goldschmidt and A.J. Pollock strung together five hits in the inning.

David Peralta led off with a single. Escobar, who had a season-high three hits, doubled. Ahmed and Kelly had RBI hits. Adam Jones gave Arizona the lead with an RBI single of his own.

“You give up three runs early to this type of team and answer with three in the bottom half of the inning, I think that shows volumes about how tough we are and how prepared we are and our character,” D-backs manager Torey Lovullo said.

Unfortunately, the D-backs’ bullpen couldn’t hold the lead.

Again the culprit was Yoshihisa Hirano, who has allowed runs in three of his four outings this season.

Hirano walked Jackie Bradley Jr. to start the seventh inning and made it just three hitters before being pulled by Lovullo.

Archie Bradley did well to not give Boston the lead and he and Greg Holland gave the D-backs a chance to win the game in the ninth.

And what an ending it was.

Escobar singled with one out and advanced to second on a passed ball before Ahmed’s would-be game-winner and Kelly’s actual game-winner.

Boston’s now 2-8 on the season and the D-backs can grab a series sweep with a win on Sunday.

“It was a great win,” Lovullo said. “We had the big ninth and it was a great moment for this team, especially Carson Kelly.”

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