Yuan captured a 6-3, 7-6(5) victory over the United States' Taylor Townsend, while Wang ousted Colombian Emiliana Arango 6-2, 6-2. Yuan rallied from a 4-1 deficit in the second set of her match to force the tiebreaker, where she then raced out to a 5-0 lead.
Wang saved nine of 10 break points in her victory. She will next face Yuan in the quarter-finals.
In other action, top-seeded Ukrainian Anhelina Kalinina beat Camila Osorio of Colombia 6-1, 6-2, and French fifth seed Diane Parry topped Russia's Kamilla Rakhimova 6-3, 6-3.
Cymbiotika San Diego Open
Great Britain's Katie Boulter came from behind to upset second-seeded Brazilian Beatriz Haddad Maia 3-6, 6-3, 6-4 in the second round at San Diego.
Boulter overcame nine double faults to advance to the quarter-finals.
In other Wednesday action, top-seeded American Jessica Pegula had no trouble with Germany's Jule Niemeier, prevailing 6-0, 6-4.
Australia's Daria Saville dropped the first set but battled past Germany's Tatjana Maria 5-7, 7-6(2), 6-2.
Maria failed to take advantage of a match point at 4-5 in the second set. After falling behind 2-0 in the subsequent tiebreaker, Saville rattled off seven straight points to extend the match.
Marta Kostyuk of Ukraine, the No. 6 seed, flew to a 6-4, 6-3 win over Australian Taylah Preston.
