As Giants quarterback Eli Manning walked off the field at Texas Stadium yesterday, he wore on his face a look of determination, not of celebration. Manning knows he hasn’t accomplished his ultimate goal just yet.

But after a 21–17 victory over the Dallas Cowboys, Manning and the Giants are just one win away from the Super Bowl, an outcome that far surpasses anything anyone outside the Giants’ locker room thought they would do this year. It was an impressive, business-like win in which the Giants looked like a good football team, not like scrappy underdogs, and it was a win that will send them to Green Bay to take on the Packers in the NFC Championship on Sunday.

It was also a win in which the Giants set the tone with their sixth offensive play, a play on which Manning took the snap, lunged forward up the middle and plowed forward for a yard, picking up a first down on third-and-1. One-yard gains on quarterback sneaks aren’t particularly exciting plays, but that was the first of many plays on which the Giants did just enough to win.

The play immediately following that quarterback sneak was the first of many plays on which the Cowboys did just enough to lose. Amani Toomer caught a Manning pass at the 40-yard line, broke the tackle of Cowboys linebacker Greg Ellis and cornerback Anthony Henry and raced into the end zone for a 52-yard touchdown to give the Giants a 7–0 lead. Poor tackling was a big problem for the Cowboys.

The Cowboys’ biggest problem, however, was the Giants’ defense. Early in the game, Cowboys running back Marion Barber ran wild, picking up 101 yards on 16 carries in the first half. But in the second half, the Giants changed their defensive approach, attacking Barber much more aggressively. Barber had 28 yards on 11 carries in the second half. That second-half defense is the kind of defense the Giants will need to play in Green Bay on Sunday, when they’ll face Packers running back Ryan Grant, who ran for 201 yards in Green Bay’s victory over the Seattle Seahawks on Saturday.

The defense may struggle to play effectively against Packers quarterback Brett Favre on Sunday, because injuries have hit the Giants’ secondary hard. One starting cornerback, Sam Madison, missed yesterday’s game with an abdominal injury, and the other, Aaron Ross, left yesterday’s game with a shoulder injury. If both starting cornerbacks are out against the Packers, Favre could pick the Giants’ secondary apart.

But Manning might just be ready to pick the Packers’ secondary apart. Yesterday Manning looked poised and under control in completing 12 of 18 passes for 163 yards, with two touchdowns and no turnovers. The Giants’ ever-improving receiving corps also played well, both the 33-year-old Toomer, who has looked rejuvenated in recent weeks, and the 22-year-old Steve Smith, who caught four passes for 48 yards and appears to have found a his niche as a slot receiver at the end of a mostly disappointing rookie year.

Manning was looking for Smith — and looking away from his favorite receiver, Plaxico Burress, who caught just one pass — in large part because Smith was usually covered by cornerback Jacques Reeves, who is the weak link in the Cowboys’ secondary. It’s a sign of the maturity of Manning that he is no longer forcing the ball to Burress and instead is finding the best matchups. Smith rewarded him with some big plays, including an outstanding catch on which he jumped for the ball and took a hard hit in the back from Reeves while he was in the air but still held onto the ball for a 22-yard gain. Although the hit from Reeves was brutal, Smith popped right back up and caught a pass on the next play.

The Packers’ greatest weakness is that their pass defense gives up too many big plays to opposing slot receivers, and that means the Manning-to-Smith connection is maturing at just the right time. The Giants will face a hostile environment at Lambeau, where the Green Bay fans will make it tough for the Giants’ offense to call its signals, and where the weather forecast calls for a high temperature in the single digits, but if the Giants’ offense plays on Sunday like they’ve played the last four weeks, they’ll score a lot of points in Green Bay.

For the first time in his career, Manning has advanced deeper into the playoffs than his older brother Peyton. Eli has a long, long way to go before he’s the kind of player Peyton is, but he took a big step yesterday and could take another one in Green Bay. The Packers crushed the Giants, 35-13, when the teams played during the regular season, and the Giants will be underdogs in Green Bay in the NFC Championship. But these Giants, who just a few weeks ago looked like they had packed it in and were heading for a Mets-like collapse, now really look like they could get to the Super Bowl. They’re one game away.

Mr. Smith is a writer for FootballOutsiders.com.