SOUTH BEND, Ind. -- By now, USC quarterback Caleb Williams is used to owning every stage he steps on. But as Notre Dame fans jumped over walls and sprinted onto the field in celebration following the Fighting Irish's 48-20 win over their longtime rivals Saturday night, Williams put his head down and made a beeline for the tunnel. Fans shouted at him, and others gleefully recorded videos of his exit. Some even tried to approach him. Williams barely flinched. After one of the worst games of his college career, all the reigning Heisman Trophy winner wanted to do was exit.

No. 10 USC had started the season 6-0 despite struggling to hold on against Arizona and Colorado the last two weeks, but the bill came due for the Trojans against No. 21 Notre Dame on Saturday night. While the defense that had struggled all season did its best to hold its own and show improvement, Williams appeared to be a shell of his usual self, and the offense he usually leads to high scores appeared to hit a wall.

"I made mistakes tonight that I usually don't make," Williams said postgame. "I've been in college for three years now and don't think I've ever had a season or game or anything like that."

Williams admitted he might have forced a few throws, but he wasn't the lone culprit. USC's shaky offensive line turned into a sieve that put Williams on his back foot from the get-go. When made uncomfortable in the past, Williams thrived by scrambling, turning dead plays into highlights. That was not the case Saturday night.

"Games like this happen in careers," Williams said. "You got to get through it. You got to keep fighting, you got to be a leader. It starts with the head of the snake, and I'll be better."

On the first drive of the game, Williams was pressured and threw only his second interception of the season. After the Trojans struggled to put three points on the board, Williams had another pass picked off when he was again pressured and his throw was tipped at the line of scrimmage. Then, just a drive later, Williams rolled out to his left like he has done plenty of times before, and instead of unleashing some magic, he dropped the ball into triple coverage where it was promptly picked.

"He is one of the best college football players that I've ever seen," Notre Dame coach Marcus Freeman said. "All I kept telling the guys is ... on this play, you have to attack. You cannot play passive, cannot play a three-man rush and let him go out and be the Heisman Trophy winner. You have to attack."

Linebacker JD Bertrand echoed Freeman, noting that the Irish defense let Williams roam far too much last season in a game that more or less clinched his Heisman win. This time around, they had one directive: Keep him in the box.

"Last year, we really experienced him getting out, him playing backyard football where he's scrambling, he's holding his play, making it 10 seconds long and, at that point, it's hard to cover downfield and receivers are getting open," Bertrand said. "So that was a big emphasis. As you could see, we tried to contain him as much as we could in the pocket."

Each of Williams' three interceptions led to an Irish score, which put the Trojans in a hole they never climbed out of. When Notre Dame drives were starting at the USC 12-yard line, the USC 2-yard line and the 50-yard line, the defense -- even on its best day -- had little to no chance. Take away those 21 points off turnovers, and the much-maligned unit under defensive coordinator Alex Grinch surrendered only three points in the first half.

"I thought our defense did enough to win the football game," USC coach Lincoln Riley said. "We put our defense in some terrible positions."

The defense began the second half with back-to-back stops of the Irish, opening the door for a USC comeback. But while Williams was able to engineer a touchdown drive to cut the deficit to 11 points, Notre Dame returned the serve with a scoring drive of its own that included a 43-yard touchdown pass from Sam Hartman -- a reminder that the USC defense was still prone to allowing big plays.

And when USC again cut the lead to 11 points early in the fourth quarter, it was time for USC's special teams to contribute to the loss by allowing a 99-yard touchdown return, the backbreaker that put the game out of reach for good.

"We kind of just took our turns making mistakes, and that's what it looks like," Riley said.

By the time the clock hit zero, Williams had been sacked six times, the Irish had forced five turnovers (including two fumbles in the fourth quarter) and USC had tallied nine penalties for 75 yards. It was as if every glimpse of a weakness the Trojans had shown over the previous six games was exposed in the span of 60 minutes.

Riley said he was "incredibly disappointed" by the result but that the team was not defeated or demoralized. He said he reminded the team in the locker room that they were still undefeated in conference play, giving them the opportunity to summon something akin to what they did last season, when a midseason loss at Utah put a road bump in their season. In his first year as USC coach, Riley still got the Trojans to the Pac-12 title game, where a win would have put them in the College Football Playoff. Utah, however, beat them once again.

As USC leaves South Bend with a loss, what awaits isn't just another rematch against Utah at home next week, but a gauntlet that includes juggernauts Oregon and Washington, as well as UCLA to finish out the season.

"The good that you see from this football team is good enough to beat anybody," Riley said. "But we obviously know we've got to put it together and put it together quickly. ... We got to let [go of] the disappointment of not playing very good tonight. We got to get past it. We got to move on."