ST. LOUIS -- Few players in baseball needed a hit more than Pete Alonso.

As Alonso stepped to the plate in the fifth inning Tuesday at Busch Stadium, he was deep in a 1-for-32 slump that had dragged his average down to .201. His slugging percentage had fallen more than 100 points. Two more hitless at-bats would have sunk him below the Mendoza Line -- not an ideal situation for anyone, let alone a cleanup hitter in a contract year. Alonso had plenty of reason to want to bust out of this slump as quickly as possible.

When he did, hitting a tiebreaking, two-run double that one-hopped up against the right-center-field fence, Alonso stood on second base and pointed both index fingers toward the visiting dugout. His teammates clapped and celebrated. Alonso needed that one. And the Mets ultimately made it stand tall, fending off the Cardinals in a 7-5 win.

“It was really nice to be able to help in the manner that I did tonight,” Alonso said.

After José Buttó allowed a three-spot in the first inning, the Mets fought back in the fifth with six consecutive hits -- the third of them a game-tying, three-run homer from Brandon Nimmo, who has recently busted free from a slump of his own. Three batters later, Alonso came to the plate with another pair of teammates on base.

For the previous two weeks, Alonso’s at-bat log had been an unending scroll of popouts, flyouts, groundouts and strikeouts. So lost was Alonso that manager Carlos Mendoza felt it necessary to give him a half-day off in Monday’s series opener, at least partially as an opportunity to clear his head.

Perhaps that was the trigger Alonso needed. Perhaps he was simply due. Either way, he launched a Miles Mikolas sinker deep into the right-center-field gap, where it landed on the warning track. Rounding third base, Starling Marte windmilled his arm, imploring Francisco Lindor to score behind him.

“At some point, he [was] going to come out of it,” Mendoza said. “He’s too good of a hitter, too good of a player for this to keep going.”

Nor was Alonso done. In the top of the ninth inning, he launched a solo homer off rookie Chris Roycroft to finish with his first three-RBI game in nearly a month. It was Alonso’s ninth home run of the season and the 201st of his career, giving Adam Ottavino some cushion to navigate a hairy bottom of the ninth.

“Great to see him have a really good game for us today,” Mendoza said.

Not all slumps are created equal, and this one was particularly trying for Alonso. Unlike Nimmo, who routinely hit the ball hard throughout his downturn, Alonso did nothing of the sort. From April 28 through May 6, Alonso ranked 279th out of 283 qualified Major Leaguers in mean exit velocity, putting the ball in play at an average of 79 mph. His career mark is almost 10 mph harder.

With the help of hitting coaches Eric Chavez and Jeremy Barnes, Alonso went into the lab, in his words, “working on different feels, just trying to get back to hitting the ball to the middle, to the big part of the field.” While one game isn’t enough to declare his work a success, Alonso hit three balls over 100 mph on Tuesday, including his homer (109.0), his double (107.5) and a sixth-inning lineout (105.9).

His efforts, along with those of Buttó, Nimmo and a makeshift bullpen missing heavily worked closer Edwin Díaz, allowed the Mets to secure their first series win since April 19-21 in Los Angeles. That also set them up to try for their first sweep since an April 15-17 set against the Pirates.

If Alonso performs the same way in Wednesday’s series finale that he did in his return to the starting lineup on Tuesday, such quests will become quite a bit easier to achieve.

“He wants to win,” Mendoza said. “If he’s 0-for but we’re shaking hands at the end of the day, that’s all that matters to him and to a lot of those guys. He shows up the next day with a smile on his face, grinding, working at his craft. For him to have a game like that, it was good.”