Avoiding a letdown after one of the biggest wins in school history was the focus all week. Finding a way to finish games better was the goal all year.

On Saturday night, Stanford football failed to do either.

In front of an electric crowd of 93,607 at the Coliseum, it was USC (8-3, 5-2 Pac-12) who took No. 4 Stanford (8-2, 6-2) into deep water and then used a late field goal to stun the Cardinal in a 20-17 upset that ended with USC fans storming the field. The revitalized Trojans toppled the Cardinal’s reign as king of California, snapping its 14-game winning streak in contests played against in-state teams. Suffering its first defeat at the Coliseum since 2005, Stanford saw its four-game winning streak against the Trojans come to an end — no Cardinal player on the roster had ever lost to USC before Saturday night.

As for the postseason picture, Stanford is officially knocked out of the national championship hunt and no longer controls its own Rose Bowl destiny, although a BCS berth is still possible.

Haunted by a shaky start, questionable play-calling and fatal mistakes in the red zone, the Cardinal looked far from the top-five team that dominated Oregon just over week ago.

“We didn’t make enough plays on offense,” said head coach David Shaw. “And when we did make plays, they didn’t result in points. Against a good team like this, playing at the [Coliseum] with all the energy and the crowd, you got to get points.”

In an ominous start, the Cardinal’s first drive of the game — a three-and-out — was plagued by a false-start penalty sandwiched between two timeouts taken by the offense due to communication issues and the crowd noise, and a costly third-down drop by junior wide receiver Ty Montgomery on a deep ball that would have resulted in a huge gain, if not a touchdown. Junior quarterback Kevin Hogan looked downright rattled.

Then the Trojans offense came out and punched the Cardinal right in the mouth.

Quarterback Cody Kessler hit his two dangerous targets, wide receivers Nelson Agholor and Marqise Lee, to convert a pair of third downs before Kessler found his wide-open fullback in the end zone for a 1-yard touchdown pass.

Even though Andre Heidari missed the extra-point attempt, it became clear that the offensive futility associated with USC’s offense early in the season was long gone. Early on, Kessler bore more resemblance to the 2011 version of former Trojans quarterback Matt Barkley than a first-year starter, and he would finish the game 25-of-37 for 288 yards and a touchdown.

The Cardinal offense regrouped on its next possession, when senior running back Tyler Gaffney broke loose for a 35-yard touchdown. It appeared as if he was going to be easily wrapped up in the backfield, but the senior somehow escaped a swarm of USC defenders and bounced to the right before making a cutback to find the end zone and put the Cardinal up by a point.

“You don’t ever assume you’re stopped as a ball carrier,” Gaffney said. “You keep going until the whistle blows — sometimes even shortly after, you’re still going.”

A week after a record-breaking 45-carry performance against Oregon, the senior showed no signs of wear and tear as he wound up carrying the ball 24 times for 164 yards and two touchdowns behind an offensive line that opened up large running lanes.

Still, the Trojans continued to move the ball at will in the first half. The Cardinal could not get a consistent pass rush on Kessler, and, despite not being known for his mobility, Kessler consistently moved out of the pocket and made great throws on the run. Halfback Javorius Allen punched it in from one yard out — Kessler found Lee in the back of the end zone for the two-point conversion — and Heidari hit a 23-yard field goal to put the Trojans up by 10.

Following three-and-outs by both teams, the Cardinal offense drove down the field with a balanced mix of runs and passes. But on third-and-goal at the USC 10, Hogan threw behind Montgomery on a slant, and Stanford settled for a field goal to make it a seven-point game at the half.

“[USC] played great in the first half, which we knew would happen,” Shaw said. “We talked about it all week: These guys were going to come out like gangbusters…and we withstood it.”

The Cardinal defense locked down in the second half as it shut down the Trojans’ rushing game — USC had just 23 yards on the ground — and finally applied constant pressure on Kessler.

“The biggest thing that changed was…our [defense’s mentality],” said fifth-year senior outside linebacker Trent Murphy, who was the Cardinal’s best defensive player on the night with eight tackles, four of which were tackles for loss.

Gaffney tied the score up early in the third quarter with an 18-yard run on third-and-8, but a defensive battle ensued from there on out.

Just like it failed to do so against Arizona State, Oregon State and Oregon, the Cardinal could not close out the game well. And on Saturday night, the inability to finish finally cost the team a victory.

The collapse started with the Trojans blocking a 30-yard field goal attempt by sophomore kicker Conrad Ukropina, who was filling in for an injured Jordan Williamson.

“That was the story of the game — missed opportunities,” Shaw said.

Stanford threatened to take the lead early in the fourth quarter when it had a first-and-goal at the USC 6. In perhaps the most controversial call of the game, Shaw elected to dial up a Wildcat play with Hogan out of the game and no pre-snap movement. The result was a 4-yard loss by Gaffney. Bad turned to worse when Shaw called the same exact play that led to the Montgomery incompletion right before the half, and, this time, safety Dion Bailey made Stanford pay by jumping the route and coming up with an interception.

“I take full responsibility,” Shaw said. “It was a bad call by me.”

Even as the Cardinal defense held its ground, Hogan made a fatal mistake with three minutes left in the game when he heaved the ball in desperation as he was getting dragged down. Just two passes after getting picked off by Bailey, Hogan was intercepted once more at the Trojan 44.

After shutting down the USC offense for the entire second half, the Cardinal defense couldn’t quite make the final big play. Playing with a short field after the Hogan interception, Kessler connected with Lee on fourth-and-2 to move the chains, and, a few plays later, Heidari hit the game-winning 47-yard field goal with 19 seconds left to pull of the stunning upset.

“It’s not a good feeling,” Gaffney said. “It’s a tough pill to swallow.”

Stanford looks to regroup for Big Game, as Cal pays the Farm a visit next Saturday. Kickoff is scheduled for 1 p.m.

Contact George Chen at gchen15 ‘at’ stanford.edu.