The Anteaters lost 80-79 in overtime on Tuesday night after giving up a two-point lead in the final 54 seconds of regulation before Will Davis II missed the key free throws.
"We let one slip away," said Michael Wilder, who led Irvine with 23 points—one off his career high—and tied his career best with six 3-pointers.
Irvine (1-1) led by four points early in the second half and then traded one-point leads with the Bruins (2-0) for a stretch. UCLA led by five when Wilder scored 10 straight points, including three 3-pointers, and Alex Young hit a 3 to put Irvine back in front, 67-65.
But Jordan Adams, who led UCLA with 26 points, made two free throws to force OT tied at 67. The game was tied three times in the extra session, with Daman Starring and Wilder hitting consecutive 3-pointers to give Irvine a 79-78 lead with 22 seconds left. Larry Drew II scored the game-winning basket for the Bruins.
The Anteaters could only look ahead to the rest of their season after falling to 10-35 all-time against Pac-12 teams.
"This is going to help us with other teams," Aaron Wright said. "We won't face a team this strong. Even though we lost, we're going to keep fighting and play even harder our next time out."
Adams made all 16 of his free throws in UCLA's first regional game of the Legends Classic.
David Wear added 13 points and nine rebounds, and his twin brother Travis had 12 points and eight rebounds for the Bruins. They will host James Madison on Thursday night in their second regional game before moving on to the championship rounds next week in Brooklyn, N.Y.
Davis added 12 points and Starring had nine points and 10 rebounds for the Anteaters.
"Every guy in my locker room came here thinking we were going to win the game," Irvine coach Russell Turner said. "Maybe we're crazy but that's important, that's how you become a winning team. We didn't get a couple of breaks maybe, but in this game the breaks will come back to you if you keep working hard. We're going to get there."
In the five-minute OT, Adams scored six points, coolly making four free throws when UCLA twice clung to 1-point leads.
Adams led UCLA in scoring for the second straight game as perhaps the least heralded of the school's highly touted freshmen. He scored 21 points in a season-opening win over Indiana State. Fellow newcomer Shabazz Muhammad, declared ineligible by the NCAA last week, spent his 19th birthday watching from the bench.
UCLA guard Tyler Lamb missed the game with a swollen left knee, a result of his recovery from arthroscopic surgery on that same knee last month.
The Bruins started strongly only to let up and see their 12-point lead shrink to 36-34 at halftime. The Wear twins combined to score 12 in a row and push UCLA's lead to 19-10 before both teams went through cold stretches. The Anteaters missed on seven straight possessions and the Bruins missed four in a row.
Irvine closed the half on a 19-9 run, with Wilder scoring five points and Flowers making four straight free throws.