Back and forth the rivals went over the final minutes, trading shot for shot and gesture for gesture.
UCLA's Thomas Welsh put the Bruins ahead by two and held his hand high in the air, wiggling his fingers to entice the sellout crowd to increase the volume inside Pauley Pavilion.
USC's Elijah Stewart countered with a three-pointer before placing an upright finger over his mouth to shush the deafening din.
An entertaining and dramatic battle for L.A. ultimately belonged to Welsh and the Bruins on Saturday after the 7-footer scored his team's final five points, lifting UCLA to an 82-79 come-from-behind victory that was essential for its NCAA tournament hopes.
Welsh repeated his finger-wiggling with both hands after sinking a three-pointer off an assist from Aaron Holiday that gave the Bruins an 80-77 lead with 52 seconds left.
"Aaron did a great job of creating a little space," Welsh said, "and I just did my best to knock it down."
The Trojans momentarily closed to within one and then, down by three, had a chance to tie in the final seconds before Jordan McLaughlin and Jordan Usher both misfired on three-pointers.
"Good final play call," McLaughlin said, "and just missed it."
UCLA, 16-7 overall and 7-4 in Pac-12 Conference play, celebrated its third consecutive victory — as well as a third straight win over the Trojans going back to last season — after getting the stops it needed to wipe out a nine-point deficit in the final eight minutes.
USC fell to 17-7 and 8-3 after its first defeat in nearly a month. Stewart's 20 points and Chimezie Metu's 11 points and 12 rebounds weren't enough as the Trojans' six-game winning streak was snapped.
It took a few superlative efforts by the Bruins. Holiday logged 23 points, nine assists and six rebounds while playing all 40 minutes and limiting McLaughlin to five points on two-for-eight shooting. Welsh, wearing a protective mask, finished with 21 points and eight rebounds in an early start to his birthday celebration.
"That sucker's unbelievable," UCLA coach Steve Alford said of Welsh. "Great birthday present for him. He turned 22 today and gets 21 and eight wearing a mask. It's just amazing."
UCLA fans from the crowd of 12,837 were roaring midway through the second half when new football coach Chip Kelly was shown on the video board during a timeout while the Bruins held a 58-55 lead. USC quickly quieted the crowd with nine consecutive points, including a vicious two-handed dunk by Metu off an inbounds lob from McLaughlin.
A "Let's go Trojans!" chant broke out from one corner of the arena after USC extended its lead to seven before being drowned out by boos. The Trojans went ahead 69-60 when Stewart made a driving layup that seemed to put UCLA's season on the brink with five of its last seven games on the road.
But the Bruins rallied with an 11-0 run that included eight points from Holiday and a tussle between USC forward Nick Rakocevic and UCLA counterpart Alex Olesinski after Olesinski flung Rakocevic to the ground near the Trojans' basket. Olesinski was assessed a flagrant-1 foul and Rakocevic a technical foul.
Rakocevic, a 57.5% free-throw shooter, was forced to take the foul shots USC was awarded. He missed both. UCLA chose Holiday to take its technical foul free throws and he made both to pull the Bruins into a 69-69 tie.
"The part that I liked," Alford said, "was I got to pick who shot the free throws."
The teams traded baskets until Welsh's three gave the Bruins an 80-77 advantage in the final minute. But Stewart made two free throws and the Trojans had a chance to go ahead after UCLA's GG Goloman missed the front end of a one-and-one with 30 seconds left.
USC called time out to set up a play but Shaqquan Aaron, who scored 12 of his 14 points in the first half, missed a jumper with 11 seconds left and Welsh grabbed the rebound before being fouled.
Welsh said he tried to keep his mind blank as he made the long walk to the free-throw line.
"Don't try to think about it," he said of his mental approach, "just step up and knock them down."
Knock them down he did, extending a lead and keeping a season upright.
UCLA UP NEXT
Thursday at Arizona, 7 p.m. PST. TV: ESPN — The Bruins defeated the Wildcats on Arizona's senior day last season and infuriated coach Sean Miller by calling a late timeout, a gesture Arizona returned during a victory over UCLA in the Pac-12 Conference tournament.
USC UP NEXT
Thursday at Arizona State, 8 p.m. PST. TV: ESPN2 — The Trojans have lost five consecutive games at Arizona State dating to 2012, by an average of 4.4 points.
— Lindsey Thiry