NEW YORK — Michael Conforto homered twice and had a career-high four RBIs, including three on two hits during the Mets’ biggest first inning in 13 years, in a 9-3 rout of the Padres on Tuesday night.

Lucas Duda broke out of a slump with three hits and three RBIs, and manager Terry Collins earned his 500th victory with the Mets.

Conforto had a leadoff homer and a two-run single in a seven-run first against Jhoulys Chacin.

Given a huge early cushion, Matt Harvey won his first home start since getting suspended. Harvey (3-3) labored through five innings to win for the first time since April 11.

There was light applause and little reaction from the crowd of 24,337 when Harvey was introduced. But there were groans when he had trouble throwing strikes with a large lead.

Chacin (4-4) got only two outs in the shortest and worst of his 145 career starts over nine seasons. He gave up seven runs and eight hits.

The Mets’ first four batters and seven of their first eight got hits. Conforto hit his fourth leadoff homer on the 10th pitch of his at-bat and capped the outburst with a two-run single that chased Chacin.

The bottom of the first took 24 minutes. It was the Mets’ biggest first-inning output since scoring seven runs in the first against the Braves on Sept. 16, 2004.

Conforto became the first leadoff hitter in team history to have three RBIs in the first inning, according to the Elias Sports Bureau. He also homered off Craig Stammen in the fourth, powering the Mets to only their third victory in 11 games.

The Padres, with the worst record in the majors at 16-31, have dropped six of seven.