The second-seeded Italian Errani rolled over Mandy Minella of Luxembourg 6-1, 6-2 in a little over an hour on the hardcourts at Sierra Madre Tennis Club.
Neither participant recorded an ace in the match, but Errani saved 2-of-3 break chances and converted on 6-of-11 chances to break Minella's serve to move on to the semifinals.
Errani will face off against Hungary's Timea Babos in the semis.
The 18-year-old Babos committed six double faults in the match, but converted on 7-of-9 break chances en route to a 6-4, 6-2 straight-set victory over Austria's Patricia Mayr-Achleitner.
Arn, the tournament's sixth seed, waited out a nearly three-hour rain delay in the second set to dispatch Nina Bratchikova of Russia, 6-1, 7-6 (8-6). The Hungarian converted on 6-of-10 break chances in the match that saw 93 minutes of on-court action.
Arn's opponent in the semis will be Romanian Alexandra Cadantu, who posted an upset by dousing eighth-seeded Spaniard Lourdes Dominguez Lino 6-2, 6-0.
This week's champion will claim $37,000.