Diaz, who has held the lead through all three rounds, is at 12-under-par for the tournament, one shot clear of Rosie Jones, who is seeking her third victory in this event.
Silvia Cavalleri of Italy stayed in contention for her first tour title with a 2-under 70 and is alone in third, three shots behind. Sherri Steinhauer, Michelle Estill and Alison Nicholas are five shots back at 209.
Diaz had four birdies and two bogeys on the front nine Saturday. Many of the players in the field had problems on the Corning Country Club greens, however, Jones needed only 25 putts on the round, eight fewer than Diaz.
Jones started the day three shots off the lead, but carded four birdies on the front nine to vault into contention.
She went to 11-under-par on the par-5 12th and barely missed the chance for another birdie on 14 when her putt slid just past the cup.
Full-field scores from the LPGA Corning Classic