Jordin Canada and Monique Billings added 10 points apiece for the sixth-seeded Bruins (13-17), who advance to a quarterfinal matchup Friday with No. 3 seed and 19th-ranked Stanford.
Fields scored all the points in a 7-1 run that put the Bruins up 58-42 with 9 minutes 18 second left. Arizona (10-20) got no closer than 11 points from there.
UCLA closed the first half on a 14-4 run to build a 37-24 halftime advantage and opened the second half with an 8-2 run to stretch its lead to 19 points.
Breanna Workman had 13 points and Candice Warthen added 12 for the No. 11-seeded Wildcats. Arizona outrebounded UCLA, 40-33, but shot just 34.5%.
Washington State 66, Oregon 64: Lia Galdeira scored a career-high 34 points for the Cougars (17-13), making six of 10 shots from three-point range and 14 of 24 overall. Washington State will face second-seeded and No. 9-ranked Arizona State in the quarterfinals.
Jillian Alleyne had 18 points and 18 rebounds for the Ducks (13-17).