The loss was the first in six Tuesday games this season for UCI (23-10), ranked No. 21 by the National Collegiate Baseball Writers Assn. It was also the first defeat in four extra-inning games this year for the Anteaters.
It was the 16th victory in the last 17 midweek contests for the Bruins (23-10), ranked No. 13 by Baseball America.
It marked only the third defeat in 21 games in which UCI has scored at least four runs this season.
Pat Valaika doubled to start the 12th for the Bruins. Justin Hazard followed with a bunt along third that pitcher Race Parmenter fielded and threw 15 feet over the head of first baseman Connor Spencer to allow Valaika to score.
After a subsequent sacrifice bunt advanced Hazard to third, he scored when UCI center fielder Dominique Taylor dropped a line drive in shallow center that he appeared ready to catch.
UCLA sophomore right-hander David Berg worked four perfect innings to collect the victory, his fourth in as many decisions this season.
Parmenter, who appears to have been displaced at closer by Matt Merten, absorbed the loss to fall to 1-2.
The Anteaters took a 4-2 lead into the ninth inning, but with one out against Merten, UCLA pinch-hitter David Williams sliced a double over newly inserted left fielder Justin Castro's extended glove. The ball appeared playable, but Castro hesitated while tracking it, and then failed to haul it in.
Junior left fielder Brandon Allen followed by lifting a two-run, game-tying homer over the right-field fence. It was Allen's first home run in 64 career at-bats.
UCI took a 3-0 lead in the first, when senior designated hitter Ronnie Shaeffer doubled in two runs and later came around to score on a sacrifice fly by sophomore catcher Jerry McClanahan.
Sophomore third baseman Taylor Sparks reached second base on a dropped pop-up to start the Anteaters' fourth inning and advanced to third on McClanahan's sacrifice bunt.
After Scott Gottschling walked, Shaeffer scored from third when Gottschling broke for second and drew a throw through, only to return to first.
UCLA used a leadoff walk to score its first run in the third, then pushed across another run with the benefit of four singles in the sixth.
The Bruins threatened in the 10th, when Eric Filia singled to open the frame and advanced to third on a one-out single. But when Filia tried to score on a pitch in the dirt that bounced about 10 feet past McClanahan, the UCI catcher retrieved the ball and threw to Race Parmenter covering home. Parmenter applied the tag, then induced a flyout to end the rally.
UCLA had runners at first and third with two outs in the 11th, but Parmenter struck out Filia to protect the deadlock.
Sparks went two for five to pace UCI's six-hit attack.
Parmenter, Jimmy Litchfield, Matt Whitehouse and Sam Moore combined for six innings in which UCLA did not manage an earned run.
UCI starter Evan Brock allowed one earned run on three hits and two walks in two innings.
Kyle Hooper worked 3 1/2 relief innings and gave up one run on four hits.
Brian Carroll, Kevin Kramer, Filia, Valaika and Williams all had two hits apiece for the Bruins, who got at least one hit from all nine spots in the lineup.
UCLA 6, UC Irvine 4 (12 innings)
SCORE BY INNINGS
UCLA 001 001 002 002 – 6 14 1
UCI 300 100 000 000 – 4 6 2
Poteet, Deeter (6), Weiss (7), Berg (9) and Zeile; Brock, Hooper (3), Litchfield (6), Whitehouse (8), Merten (9), Parmenter (9), Moore (12) and McClanahan. W – Berg, 4-0. L – Parmenter, 1-2. 2B – Rabago (UCI), Shaeffer (UCI), Zeile (UCLA), Williams (UCI), Valaika (UCLA). HR – Allen (UCLA) .