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SAN FRANCISCO -- The New York Giants have their own Super Bowl formula: in overtime and on the road.

And with Lawrence Tynes' foot.

Five plays after the 49ers' Kyle Williams fumbled a punt, Tynes kicked a game-winning 31-yard field goal in overtime, sending the Giants to the Super Bowl with a 20-17 victory over San Francisco in the NFC Championship Game on Sunday.

In another tight one in this decades-old postseason rivalry, both defenses made key stops before New York capitalized on a rare mistake in San Francisco's resurgent season. Williams' blunder put the Giants in perfect position for another sensational finish in a season full of them.

"That was a tough game. We had to fight for every yard that we got," Giants quarterback Eli Manning said. "Defense was outstanding, special teams getting us two turnovers was huge. That led to 10 points."

The first three overtime series ended in punts before Williams fumbled. The Giants won it moments later and silenced -- for good this time -- the towel-waving, poncho-wearing sellout crowd at cold, rainy Candlestick Park.

"It was one of those situations where I tried to turn it upfield and it just didn't work out," Williams said.

Manning and the Giants (12-7) will face the New England Patriots in the Super Bowl on Feb. 5 in Indianapolis as 3½-point underdogs. The last time the teams met for the NFL title, 2008, the Giants ended the Patriots' bid for a perfect season.

Tynes had a hand, er, foot in getting the Giants to that one, too, kicking the game-winning field goal in overtime.

Devin Thomas put the Giants in position this time by recovering his second fumble of the game after Jacquian Williams stripped the ball from fill-in return man Williams, who also fumbled earlier to set up a New York touchdown.

"It's my second NFC Championship Game, my second game-winner," Tynes said of his kick 7:54 into overtime. "It's amazing. I had dreams about this last night. It was from 42, not 31, but I was so nervous today before the game just anticipating this kind of game. I'm usually pretty cool, but there was something about tonight where I knew I was going to have to make a kick. Hats off to Eli, offense, defense. Great win."

Holder Steve Weatherford celebrated with a slip-and-slide on his back down the soggy field. Victor Cruz fell to his knees. Tynes quickly found his crying wife for a warm hug. Manning tossed his gloves into the temporary seats with a big smile, then received a surprise visit from big brother, Peyton, in the locker room.

Manning went 32 of 58 for 316 yards and two touchdowns and overcame six sacks in his record fifth road playoff win, New York's fifth in a row overall.

Manning threw a go-ahead 17-yard touchdown pass to Mario Manningham with 8:34 remaining after Williams fumbled for the first time.

The Giants challenged that the ball touched Williams' right knee and Thomas recovered with 11:06 left and coach Tom Coughlin won, giving the Giants the ball back at the 29.

"That was a tremendous football game for those that really enjoy football at its very basic element," said Coughlin, who matched former Cowboys coach Tom Landry for most road playoff wins with seven. "Just a classic football game that just seemed like no one was going to put themselves into position to win it. Fortunately we were able to do that."

A 12-point underdog in the 2008 title game, the Giants battered Brady and got a last-minute TD pass from Manning to Plaxico Burress to win their third Super Bowl. Five months ago, Manning declared he was in the same class as Tom Brady. Now, he'll get another chance to outdo him on the NFL's biggest stage.

During this playoff run, he's already outplayed Aaron Rodgers and the defending champion Packers, and fellow former No. 1 pick Alex Smith.

Cruz set the tone Sunday with eight of his 10 receptions in the first half and finished with 142 yards.

"It's just been a tremendous effort by all of us, man," Cruz said. "We understand that any one of us can get hot at any moment. As long as we're all on the same page and just playing together, man, we've got a great group of guys."

The Giants appeared on the verge of collapsing and Coughlin's job status was in jeopardy just a month ago, when they fell to 7-7 with an embarrassing loss to the Washington Redskins on Dec. 18.

They were facing elimination the following week against the Jets and Rex Ryan, but the Giants won 29-14. They followed with a 31-14 win over Dallas in the regular-season finale to win the NFC East and get to the playoffs for the first time since 2008.

New York dominated Atlanta at home in the opening round, and then came another stunner: a 37-20 victory at Green Bay.

Vernon Davis caught touchdown passes of 73 and 28 yards for the NFC West champions (14-4), who went from 6-10 a year ago to a contender and ended an eight-year playoff drought.

"It will be a tough one. It will take a while to get over," Harbaugh said. "There were a lot of ways in which we played well enough to win. We just didn't come away with it."

Smith completed just 12 of 26 passes for 196 yards, connecting on only one short throw to a wide receiver. With no threats on the outside, San Francisco managed one third-down conversion, coming on the final play of regulation. The offense was unable to overcome Williams' blunders.

"We all know him. We know how committed he is to winning," Smith said. "It's not on him. I look at the 1 for 13 on third downs. I know he's going to feel bad, but he's still part of our team. We didn't lose the game there. We lost it across the board offensively. We just couldn't get it done."

The only other time these two franchises faced off in the conference championship the game finished in memorable fashion. On Jan. 20, 1991, Roger Craig fumbled with the 49ers leading 13-12 late in the fourth quarter and the Giants went on to win 15-13 to deny San Francisco a chance at a third straight Super Bowl title. New York then beat the Bills to capture its second Super Bowl.

This time, it was Williams who made the crucial mistake.

"You hate to be the last guy that had the ball, to give it away in that fashion and to lose a game of this magnitude," Williams said. "It is what it is. We're going to move forward as a team. Everyone has come to pat me on the back and the shoulder to say it's not me."

Game notes

Davis joined Jerry Rice as the only 49ers with at least two touchdowns receiving in back-to-back playoff games. ... Cruz caught a 36-yard pass from Manning on the first play of the second quarter, then Manning hit Bear Pascoe for a 6-yard touchdown seven plays later. ... Giants C David Baas beat his former team. ... Manningham missed the potential game-tying TD in a 27-20 loss here on Nov. 13.