Lucas Duda of the Mets celebrates his ninth-inning, walk-off, two-run...

Lucas Duda of the Mets celebrates his ninth-inning, walk-off, two-run home run against the Houston Astros at Citi Field on Sept. 27, 2014. Credit: Getty Images / Jim McIsaac

Lucas Duda broke from the script. As he rounded the bases Saturday night, the typically stoic Mets first baseman cracked a smile before completing his journey by jumping onto home plate.

"I don't usually hit walk-off home runs," Duda said, after his two-run shot with two outs in the ninth inning lifted the Mets to a 2-1 win over the Astros. "So, I don't really know what to do."

The moment continued what has been a breakout season for Duda, who has set career highs in homers (29) and RBIs (88).

Lefthanders have been Duda's Kryptonite, holding him to a .174 average coming in. But when Astros lefty Tony Sipp missed with his 1-and-0 pitch, Duda lined a drive off the rightfield foul pole.

The Mets' dugout emptied to greet Duda at the plate, where they celebrated a victory after being one out from defeat.

"It was special seeing him break out of that shell in front of so many people," said Eric Young Jr., who triggered the rally with a one-out triple.

With his parents in attendance, Duda bashed the first walk-off homer of his major-league career, and the first by a Met since Ike Davis' walk-off grand slam on April 5.

The homer took rookie Rafael Montero off the hook for a loss, even though he allowed just one run in 51/3 innings and lowered his ERA to 4.06.

Next spring, Montero, 23, will come to camp as a starter, though his chances of cracking the rotation are slim, barring a trade. "We'll see where he fits," Mets manager Terry Collins said before the game. "If we get to the end and we need help in the bullpen, we can make an adjustment. We've got to look at where he can help the most."

With a victory in Sunday's season finale, the Mets would finish with 79 victories, their most since 2010.

Notes & quotes: Outfielder Kirk Nieuwenhuis is expected to be released from the hospital Sunday after suffering what Collins called an infection. Nieuwenhuis had been scratched from Friday's game with an unspecified illness . . . Jonathon Niese will be seen by doctors on Monday, two days after leaving his start with an elevated pulse.