The Mets played at home for the first time since the All-Star break and lost in a matchup of NL wild-card contenders. Citi Field was nearly empty at the start, a day after a rainout forced the twinbill.

Carlos Martinez (10-6) gave up a two-run homer to Rene Rivera and left after the fifth inning with a 3-2 lead. Three relievers finished, with Seung Hwan Oh getting his fifth save in six chances.

Curtis Granderson led off the Mets ninth with a single, but was thrown out trying to tag up on Yoenis Cespedes’ drive that Tommy Pham caught in deep center field.

Syndergaard (9-5) has won only one of his last five starts. The All-Star had a tough sequence in the second — he got hit in the lower right leg by Pham’s one-hop single to load the bases, then made a wild, off-balance throw home on Martinez’s comebacker to let a run score.

The Mets lost despite throwing out three runners on the bases and manager Terry Collins winning a pair of replay challenges. New York has alternated wins and losses for the last 11 games.

The Cardinals opened a 10-game trip that also takes them to Miami and Cincinnati. They arrived at their New York hotel around 8:30 a.m. Monday after plane trouble, and the rainout gave them some much-needed rest.