Cincinnati ? Pedro Martinez blew ’em away. Carlos Beltran knocked ’em in. Everything was going exactly according to plan for the new-look Mets — until Adam Dunn exposed the one area they overlooked.

Dunn’s second homer of the game tied it in the ninth, and former Royal Joe Randa followed with a solo shot off Braden Looper that sent the Cincinnati Reds to a 7-6 victory Monday in front of the biggest crowd in Great American Ball Park’s history.

For the first time in their 129 season openers, the Reds finished one with a homer.

“I felt like I was at a rock concert, screaming for the next song,” said closer Danny Graves, who pitched the ninth and then piled on Randa at home plate. “We’ve had walk-off wins in the past, but I haven’t had a feeling like this in a long time. It’s the perfect script. I won’t believe it until I watch it (on television).”

The Mets couldn’t believe it, either. Not after the way Martinez and Beltran — the cornerstones of their expensive offseason makeover — performed in their New York debuts.

Martinez struck out 12 and allowed only one hard-hit ball — Dunn’s three-run homer — during his six innings. Beltran had three hits, including one of the Mets’ three homers, as they went ahead 6-3.

Martinez blew away the Reds and any doubt that at age 33 he can still dominate. After Dunn’s first homer, he struck out 12 of his last 14 batters, making them look overmatched against his 94 mph fastball and nasty breaking ball.

“Pedro was on top of his game,” Randa said. “He was carving through us. He was unhittable. Once he was out of the game, there was a big sigh of relief on the bench. Crazy things happen on Opening Day.”

Manny Aybar gave up Jason LaRue’s RBI double in the seventh. Looper then let it slip away in only 14 pitches — Austin Kearns’ single, Dunn’s homer and Randa’s final swing. The third baseman raised his fist as he rounded first base, then got pummeled by his new teammates at home plate.

Phillies 8, Nationals 4

Cincinnati's Joe Randa, left, steps on home plate for the winning run. Randa's walk-off home run lifted the Reds past the New York Mets, 7-6, Monday in Cincinnati.

Philadelphia — Kenny Lofton hit a three-run homer, and Jon Lieber pitched 5 2/3 effective innings to help the Phillies ruin the Washington Nationals’ debut. Terrmel Sledge hit the first homer in Nationals’ history and drove in three runs.

Lieber allowed three runs and 10 hits for the win. Lofton reached four times and scored twice, and Pat Burrell had three hits and two RBIs, helping manager Charlie Manuel win his debut with the Phillies.

Rockies 12, Padres 10

Denver — Rookie Clint Barmes hit a game-winning two-run homer, capping Colorado’s four-run ninth inning off Trevor Hoffman.

Aaron Miles had a career-high five hits, including an RBI single to tie it, and Jeff Baker and Preston Wilson homered for Colorado, which had 18 hits. Ryan Speier pitched two-thirds of an inning for the win in his major-league debut.

Brewers 9, Pirates 2

Pittsburgh — Ben Sheets allowed two runs over seven innings to outpitch Oliver Perez in a matchup of two of the NL’s best young starters.

Jeff Cirillo, who played for Milwaukee from 1994-99, homered and doubled for the Brewers. Rookie shortstop J.J. Hardy, known more for his defense in the minors, added a two-run single for his first career hit in a five-run Brewers sixth.

Perez allowed six runs and five hits, walked three and hit a batter in five-plus.

Cubs 16, D’backs 6

Phoenix — Aramis Ramirez went 3-for-4 with a two-run homer and four RBIs, leading a dominant performance by the Cubs’ lineup.

Derrek Lee had four hits and five RBIs, including a three-run homer and two doubles. Todd Walker, Jeromy Burnitz and Corey Patterson also had three hits apiece for Chicago, which amassed 23 hits and set a team record for runs in an opener.

Javier Vazquez lasted just 12/3 innings for Arizona.