SAN FRANCISCO — As the Giants went through their various transformations this season — from a running team to a passing team, from a weak defense to a stout one — the constant was always Eli Manning in the fourth quarter. It did not matter what had happened to the Giants early in the game; a heavy dose of Manning late was the cure for all that ailed them.

And so it was again Sunday. The Giants had not needed Manning’s magic for a month or so, not as they roughed up the Jets and Dallas, the Atlanta Falcons and the Green Bay Packers. But in the N.F.C. championship game here against the San Francisco 49ers, Manning was asked to, and did, author the most important comeback of the year as the Giants advanced to the Super Bowl with a 20-17 victory.

“Eli is just as calm in the fourth quarter as he is in the first quarter of a preseason game,” Manning’s brother Peyton said outside the Giants’ locker room afterward. “They expect to score. That’s impressive.”

Amid the postgame bedlam near the Giants’ team bus, Peyton’s smile was nearly as wide as Eli’s. The elder Manning, whose neck injury kept him sidelined the entire season and has put his status with the Indianapolis Colts into question, did not tell his younger brother that he was planning to attend Sunday’s game. He did not ask Eli for tickets and did not alert him to his presence until greeting him with a celebratory hug.