Fatigue was setting in. Faith, ever fragile, was wavering. USC’s revamped defense had held mostly tough. Its new starting quarterback looked mostly comfortable. But USC’s grasp on its season opener had been slipping away since halftime. One drive, done in by a botched trick play. Another swallowed up by a fourth-down swat.

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

Moss pumped his fists in jubilation, roaring to the highest reaches of Allegiant Stadium, his gutsy performance helping secure a season-opening, 27-20 win against an SEC opponent.

USC still needed some help after that stunning score, as its defense held deep in its own territory and Moss maneuvered the Trojans down the field for a game-clinching touchdown run by Woody Marks.

After an offseason of uncertainty and unanswered questions, Moss and the Trojans offered a resounding response against the 13th-ranked Tigers, marking the first time in his three seasons at USC that coach Lincoln Riley beat a team ranked higher than his own in the regular season.

USC wide receiver Kyron Hudson makes a one-handed catch for 24 yards in front of LSU safety Major Burns and linebacker West Weeks, left, during the first half Sunday.

(Robert Gauthier / Los Angeles Times)

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

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

LSU initially had little trouble moving the ball against USC’s revamped defense, as Garrett Nussmeier got rid of the ball quickly, neutralizing 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 critical stop.

Facing an LSU fourth down, inside the three-yard-line, USC dialed up a perfectly timed inside blitz from linebacker Mason Cobb that forced one of Nussmeier’s few errant throws.

USC’s offense took a bit longer to click after it touched the ball for barely three minutes in the first quarter.

But where LSU played it conservative with its new quarterback, Moss let it rip early and often. He hit six passes of 17 yards or more in the first half alone, the most stunning of which saw him throw into a crowd of USC defenders, only for wideout Kyron Hudson to leap up and pull it down one-handed.

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)

Hudson’s highlight-reel catch proved to be the catalyst USC needed, as Marks punched in a touchdown two plays later.

It was the only time the Trojans found the end zone in the first half, despite twice more driving deep into LSU territory. One drive, jump-started by an electric return from Zachariah Branch, stalled at the five-yard line and resulted in a field goal. Another, in the final seconds of the first half, wasn’t so fortunate.

With a minute remaining, USC let precious time tick away before kicking into high gear, driving all the way to the LSU 12-yard line. Instead of a quick shot or two at the end zone, Riley opted to play it safe, run the ball and kick the easy field goal to end the second quarter.

But Michael Lantz, the Trojans’ new kicker, missed the 29-yard attempt.

As the game grinded to a crawl in the second half, with both defenses clamping down, the missed opportunity loomed large over USC. By midway through the third quarter, LSU started to impose its will on the ground, just as Tigers left tackle Will Campbell promised earlier this week. Running back John Emery Jr. broke one long run, then two plays later, another.

It took safety Kamari Ramsey falling in coverage on third down for Nussmeier to find wideout Aaron Anderson for a touchdown and a lead that wouldn’t last.