NEW YORK – There was no certainty Pete Alonso would be returning to Citi Field after his final regular-season home game on Sept. 22. Not as a member of the Mets, at least. There was a standing ovation, a tip of the helmet … just in case.

But before the slugging first baseman, who has played his entire six-year career in New York, enters free agency this winter, he had business to handle.

“The whole goal is, that wasn’t going to be the last game for us,” Alonso said.

One week after saving the Mets’ postseason with a go-ahead, three-run homer in the clinching Game 3 of the Wild Card Series, Alonso’s opposite-field home run in his first at-bat off longtime rival Aaron Nola helped put the Mets one win away from advancing to the National League Championship Series in a 7-2 victory over the Phillies in Game 3 of the NL Division Series on Tuesday.

“He's on,” said manager Carlos Mendoza. “I think we've been saying it the whole year: He can carry a team, he can carry us, and we're here because of that big swing he got in Milwaukee. Then today, first pitch he sees off a really good pitcher in Nola, he goes the other way, and he goes with ease. As soon as he hit it, everybody in the ballpark knew that ball was gone.”

The matchup between Alonso, 29, and Nola, 31, goes back to their days at Florida and LSU. Although Nola dominated the collegiate battles, Alonso has had the edge in the Majors.

Alonso described the matchup that goes back to 2014 as a “cat-and-mouse game.” In the second inning, he walloped a first-pitch, 94.4 mph fastball from Nola 385 feet to right field to put the Mets on the board. Nola didn't execute on his plan “trying to go down and away.”

“Every time, it’s just trying to crack a code: How is he trying to get me out today? What’s he trying to get ahead with?” Alonso said of Nola. “He’s got really pinpoint control over all of his pitches. But I was just looking for a fastball out over the plate, just stay through it and drive the ball to the big part of the field.”

The solo blast marked Alonso’s sixth career home run off Nola, breaking a tie with Nationals left-hander Patrick Corbin for Alonso’s most off any pitcher. Alonso tied Braves third baseman Austin Riley for the most homers by a hitter against Nola.

“Nola knows him well; Pete knows him well, as well,” said Mendoza. “... Two really good players facing again in the Division Series, and Pete got him today.”

Alonso had not gone yard off Nola since Aug. 19, 2022. When he did so in Game 3, he channeled new-found opposite-field power.

Through Aug. 26, only two of Alonso’s 27 homers (7.4 percent) were hit oppo. Since Aug. 27, Alonso has powered five of his 10 home runs to the opposite field, including all three of his playoff blasts.

Conversely, the two-time Home Run Derby champion has hit just 10.2 percent of his regular-season career homers to the opposite field. Prior to this postseason, he had only hit one to the opposite field in the playoffs.

“Right now, I’m just trying to get a pitch I can handle and put a good swing on it and stay within myself,” Alonso said. “Wherever it goes out – if it does – I’m just happy it does. It’s just the product of a good swing, and the pitch was away. If I’m hitting balls the other way, that’s typically a good sign.”

Alonso’s offensive display prompted postgame smiles from his teammates, particularly those who went yard – or came short.

“I hit my ball pretty hard and it didn’t go out, and then he comes in and hits an oppo home run so I was like, ‘Well, I guess that’s how you do it,’” said shortstop Francisco Lindor, who flied out to deep center field in his first at-bat. “Pete’s Pete. He’s special. He’s one of the better power hitters in the game that we have had in a while. He came in, he got a good pitch, he had a game plan, he executed it, and hit a home run.”

Jesse Winker, who hit a home run to right field in the fourth inning, laughed, “It makes me realize I can’t do that, that’s for sure. It’s huge. He’s already changed games with one swing. He did it again tonight. He got us some momentum early. He’s such a talented hitter, [it’s] so much fun to watch him do his thing.”

With his opposite-field power, Alonso has the Mets trending in the right direction. Teams with a 2-1 lead in best-of-five playoff series that play Game 4 at home have won the series 81 percent of the time.

“It would be fantastic to get it done,” Alonso said. “But we've got to stay within ourselves, focus on the task at hand pitch to pitch, out to out, inning to inning. I believe if we can do that, stay right there in the groove of things and don't try to do too much in each moment, I think we'll be able to get it done tomorrow.”