Jason Marquis became the second pitcher to hit a grand slam against the Mets this season, and the Chicago Cubs clinched home-field advantage throughout the NL playoffs with a 9-5 victory tonight that damaged New York’s postseason chances.

Struggling to avoid another late collapse, the Mets slipped 2 1/2 games behind first-place Philadelphia in the NL East and had their wild-card lead cut to one game over idle Milwaukee.

Jon Niese (1-1) lasted only three-plus innings in his third major league start and New York lost its third straight since moving into first place Friday with a win at Atlanta.

Marquis (11-9) drove in five runs and pitched effectively into the seventh, a big game in his hometown with friends and family in the stands. Derrek Lee also homered and Reed Johnson had a pair of RBI singles for the NL Central champions in the opener of a four-game series that could be a postseason preview.

New York would play Chicago (95-60) in the first round if the NL standings don’t change.

But the Mets are having a hard time holding on – just like last September, when they squandered a seven-game cushion with 17 to play and missed the playoffs as Philadelphia charged past them to win the division title.

New York went 1-6 at Shea Stadium during the final week last year and got off to a foreboding start Monday night as they began another season-ending, seven-game homestand.

Once again, when the Mets desperately need wins they’re having all kinds of trouble beating teams that don’t. David Wright hit a two-run homer and Daniel Murphy had an RBI double.

Kerry Wood struck out Luis Castillo with two on for his 33rd save in 39 tries.

Marquis connected off Niese in a six-run fourth, snapping a 2-all tie with his fifth career homer and second this season. Seattle pitcher Felix Hernandez also cracked a grand slam at Shea Stadium this year, on June 23 off Johan Santana.

The 1977 Cubs were the previous team to give up two grand slams to pitchers in one season. Montreal’s Don Stanhouse and Philadelphia’s Larry Christenson hit those long balls, according to the Elias Sports Bureau.

Marquis added an RBI groundout in the fifth to make it 8-2. The last Cubs pitcher to drive in five runs was Milt Pappas on August 11, 1972, against the Mets at Wrigley Field.

Kevin Tapani hit the previous grand slam by a Cubs pitcher on July 20, 1998, at Atlanta off Denny Neagle.

The five RBIs were a career high for Marquis, who entered the game with a .205 career batting average and 35 RBIs – including five this season.

Chicago improved to 3-0 against the Mets this year after sweeping a two-game series at Wrigley Field from April 21-22.