Fatigue was beginning to set in. Faith, always fragile, was wavering. USC’s revamped defense had largely held up. The new quarterback seemed comfortable for the most part. But USC’s grip on the season opener had been slipping since halftime. One drive, thwarted by a botched trick play. Another, swallowed by a fourth-down swat.

Still, Miller Moss remained undaunted. More than nine months had passed since the quarterback’s memorable Holiday Bowl debut, long enough to seriously wonder if his six-score performance was just a bowl season mirage. But Moss never faltered, even when LSU defenders rushed him on third down in the fourth quarter and he fired an improbable pass toward the corner of the end zone, where Ja’Kobi Lane had his arms outstretched for the touchdown.

Moss pumped his fists in joy and roared to the upper echelons of Allegiant Stadium. His courageous performance had earned him a decisive 27-20 victory over an SEC opponent in the season opener. It was their first game against the conference in nearly 50 years.

USC would still need some help after that stunning score, as the defense held firm deep in its own territory and Moss maneuvered the Trojans down the field for a game-winning score. But after an offseason filled with uncertainty and unanswered questions, the Trojans and their new quarterback would provide a resounding response against the 13th-ranked Tigers, marking the first time in his three regular seasons that coach Lincoln Riley had defeated a team ranked higher than his own.

He had his new defensive coordinator, D’Anton Lynn, to thank for that, as USC’s defense limited an opponent to just two scores for the first time since last September. The Trojans still gave up 421 yards, but LSU averaged just 6.6 yards per play, a big improvement from last season. USC also tackled much better and made key stops with its backs against the wall.

Moss took advantage of those saves, completing 27 of 36 passes for 378 yards and a touchdown.

USC wide receiver Zachariah Branch avoids a tackle attempt by LSU linebacker Harold Perkins Jr. after making a catch in the first half.

(Robert Gauthier / Los Angeles Times)

LSU had little trouble moving the ball against USC’s new-look defense, as Garrett Nussmeier got rid of the ball quickly and neutralized USC’s pass rush with a barrage of short and intermediate passes. He completed 20 of 24 passes in the first half for 191 yards, leading four drives of eight-plus plays, including a 13-play opening drive that had USC on its heels … until the Trojans came up with a crucial stop.

On an LSU fourth down inside the 3-yard line, USC called for a perfectly timed inside blitz from linebacker Mason Cobb that forced Nussmeier into one of his few errant throws.

It took a while for USC’s attack to get going, after having the ball for barely three minutes in the first quarter.

But where LSU played it conservatively with its new quarterback, Moss was content to make it happen early and often. He hit six passes of 17 yards or more in the first half alone, the most stunning of which came when he threw the ball into a crowd of USC defenders, only to have wideout Kyron Hudson leap up and take him down with one hand.

Hudson’s high-profile catch proved to be the catalyst USC needed, as running back Woody Marks scored a touchdown two plays later.

It was the only time the Trojans would find the end zone in the first half, despite driving deep into LSU territory twice more. One drive, started by a hot return by Zachariah Branch, stalled at the 5-yard line and resulted in a field goal. Another, in the final seconds of the first half, was less fortunate.

With a minute left, USC wasted precious time before kicking into high gear, driving all the way to LSU’s 12-yard line. Instead of a quick shot or two into the end zone, Riley opted to play it safe, running the ball and firing the easy field goal.

USC wide receiver Makai Lemon, right, is tackled by LSU safety Sage Ryan (3) in the first half of Sunday’s game.

(Robert Gauthier / Los Angeles Times)

But Michael Lantz, the Trojans’ new kicker, missed the attempt from 29 yards out.

While the game started slowly in the second half, with both defenses holding back, the missed opportunity would be huge for USC. That’s because midway through the third quarter, LSU began to impose its will on the ground, just as its left tackle, Will Campbell, had promised earlier in the week. Running back John Emery Jr. broke off a long run, then two plays later, another.

Yet it was safety Kamari Ramsey who fell into coverage on third down before Nussmeier could find wideout Aaron Anderson for a touchdown and a lead that would not last long.