New England linebacker Junior Seau celebrates a sack of San Diego quarterback Philip Rivers. Seau, a former Charger, and the Patriots beat the Chargers, 21-12, Sunday in Foxborough, Mass., improving their record to 18-0 heading into the Super Bowl on Feb. 3.

Foxborough, Mass. ? Perfect isn’t always pretty.

And every now and then, the blemish-free New England Patriots win a game the hard way.

The Patriots are going back to the Super Bowl for the fourth time in seven seasons because of something new. Their running game and defense overcame uncharacteristic mistakes from Tom Brady and a startlingly ineffective passing game, pushing the Pats to a 21-12 triumph over San Diego on Sunday at Gillette Stadium.

New England’s defense kept the injury-riddled Chargers out of the end zone for the first time all season, limiting them to four field goals, then turned their perfect season over to running backs Laurence Maroney and Kevin Faulk.

Maroney rushed for 122 yards and a touchdown, Faulk caught a team-high eight passes and the unbeaten Pats (18-0) ground away the final 9 minutes, 13 seconds to gain a berth in Super Bowl XLII.

San Diego (13-6) got one series with All-Pro runner LaDainian Tomlinson before he decided his sprained right knee was unfit. And though Michael Turner and Darren Sproles hit New England with big plays, neither they nor sore-kneed Philip Rivers made anything happen inside the 10-yard line, a decisive factor.

“We had opportunities,” Chargers Coach Norv Turner said, “and ended up having to kick field goals.”

Instead, a Patriots team that scored more points than any team in NFL history and set a slew of franchise passing records flipped its customary game plan at halftime and let Maroney and its offensive line dictate the tone on a blustery, 23-degree day that fit the ground game.

“We can play any style of football you want us to play, whether it’s a shoot-’em-out game or hard-core physical game,” Patriots safety Rodney Harrison said. “We’re prepared for it all, because that’s what we do. We have the talent and the personnel and the smarts to play whatever game you want to play.”

The game Sunday was simple, from the Pats’ perspective. Find a way, any way, to advance.

They kept alive their shot at being the first team since the 1972 Dolphins to win every game by skirting Brady’s season-high three interceptions and escaping their second-lowest point total of the season.

With Brady fighting himself, winds that gusted up to 24 mph and, later, an unexplained gimpy right leg, the Pats did what teams try to do to them: They got a lead, escaped some danger zones and decided they could survive by shortening a low-scoring game.