Struggling to avoid a second consecutive September slide, the Mets dropped 2½ games behind first-place Philadelphia in the NL East and had their wild-card lead cut to one game over idle Milwaukee.
"It has nothing to do with being tight. It has nothing to do with last year. It has to do with we're struggling," David Wright said. "We need to just look at the big picture. There's no need to panic. Everything we want to accomplish is right out there for our grabbing and right out there in front of us."
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 after he left nearly 30 tickets for friends and family. In fact, the Cubs had a vocal band of rooters in the Shea Stadium crowd.
"This was really special," Marquis said.
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.
"Last year is over with. It's done with. It was a failed opportunity. This year we have a golden opportunity," Wright said.
But once again, when the Mets desperately need wins they're having all kinds of trouble beating teams that don't.
"When you sign on with the Mets, you sign on with being a part of what happened last year," manager Jerry Manuel said. "We have to exorcise those demons at some point, there's no question. Some big ones, too."
Wright hit a two-run homer and Daniel Murphy had an RBI double for New York. 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.
"I knew he was a good-hitting pitcher. That's a reason I threw breaking balls to him," Niese said. "Obviously, I didn't want to hang it. But I hung it and he put a good swing on it."
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 Aug. 11, 1972, against the Mets at Wrigley Field.
"Anytime you have a chance to help yourself, why not?" Marquis said. "I consider myself an athlete and I try very hard to do everything I can."
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.
Notes: Mets CF Carlos Beltran was shaken up after banging into the fence on a seventh-inning catch, but stayed in the game. His knee was swollen and sore, yet Beltran expects to play Tuesday. ... New York RHP John Maine, on the disabled list with a bone spur in his pitching shoulder, faced five batters in a simulated inning before the game. The Mets hope he might be able give them an inning or two out of the bullpen during the last few days of the season. ... The Cubs adjusted their pitching rotation to begin preparing for the playoffs. LHP Sean Marshall will start Tuesday night, bumping RHP Rich Harden back to Thursday. ... Lee gave the Cubs five players with 20 homers.
