SAN DIEGO -- Jackson Merrill is a 21-year-old rookie. He might also be the most clutch hitter in baseball.

Hard to argue otherwise, after the Padres’ phenom launched yet another crucial, ninth-inning home run against yet another elite reliever on Sunday afternoon.

Merrill’s walk-off blast against Mets closer Edwin Díaz was the difference in San Diego’s 3-2 victory at Petco Park -- a win that might prove hugely important in the National League playoff picture.

“He's that guy,” said left fielder Jurickson Profar, who had tied the game an inning earlier with a two-run homer. “He has it. … Everyone in the dugout called it.”

Not like they were going out on a limb. Even before Merrill’s latest walk-off, he’d set a record for a player age 21 or younger with the most game-tying or go-ahead home runs in the ninth inning or later (since at least 1961). He now has five of them. No one else in the Majors has more than three this year.

Clearly, Merrill has a knack for hitting big home runs in big spots. But this one might have been his biggest yet. He got ahead in the count, 2-0, when Díaz hung a slider over the heart of the plate.

“I know I'm going to get something to hit,” Merrill said. “I don't think he wants to walk anybody. I was just kind of looking for something in the middle. He gave me a little bit with the slider right down the middle.

“It just happened again.”

Remarkable, truly. Merrill has come to the plate 14 times this season with the Padres tied or trailing by a run in the ninth inning. He’s hitting .583 with a walk and five home runs. Against quite a list of relievers, too: Mason Miller, Blake Treinen, David Bednar, George Soriano and Díaz.

“He’s not making it bigger than it is,” Padres manager Mike Shildt said. “He’s just letting his talent play -- and his talent’s really, really, really good.”

The victory earned the Padres a split of their four-game weekend series against New York, and, crucially, kept the gap in the NL Wild Card race at 5 1/2 games over the Mets, who are the first team on the outside looking in. San Diego also kept pace with the Dodgers and D-backs in the NL West race after both won on Sunday.

The Padres have mostly been rolling since the All-Star break. They entered the weekend having won nine series out of 10, and they hadn’t lost consecutive games until earlier this week. In a way, Sunday brought some of the first real adversity of the second half. San Diego had dropped three of four and stared down the prospect of falling closer to the cut line with another loss to the Mets.

Then came Sunday’s response.

“It was big,” Profar said. “They've been beating us. We got swept over there in New York, and they were on the brink of winning another series against us. We had to win this game.”

Then again, Merrill didn’t necessarily see it that way.

“Another game,” he said. “There's no point in making it any bigger than it is.”

Big game or not, the Padres chased this one hard. Shildt went to his bullpen in the fourth inning, after three consecutive walks from starter Martín Pérez. Right-hander Bryan Hoeing was called upon to face MVP candidate Francisco Lindor -- and punched him out with a filthy 1-2 splitter.

“Bases loaded against Lindor, my mindset is just to get him out somehow, some way,” Hoeing said.

Even though the Padres trailed, Shildt used Jeremiah Estrada, Jason Adam and Tanner Scott -- his three high-leverage setup options -- for the sixth, seventh and eighth innings, respectively.

That aggressiveness paid off. Newcomer Mason McCoy worked a one-out walk in the bottom of the eighth, setting the stage for Profar. Mets reliever José Buttó threw a fastball into Profar’s sweet-spot -- thigh-high on the outer half. Profar launched it into the right-field seats, then flipped his bat and fired up the Padres’ dugout.

“We were never out of the game,” Merrill said. “It was only 2-0. We were right there the whole time. ... The homer just added it right to our minds: ‘We're winning this game.’”

An inning later, Merrill made sure of it.