Gauff overcame six double faults to defeat Andreescu 6-4, 4-6, 6-3.
Gauff will face No. 1 seed Iga Swiatek, the Polish player who also needed three sets to dispatch China's Qinwen Zheng 6-4, 4-6, 6-1.
Madison Keys upended No. 8 Daria Kasatkina of Russia 6-4, 6-3; third-seeded Aryna Sabalenka rallied to defeat Sloane Stephens 1-6, 6-3, 6-2 in 2:01; and No. 2 seed Paula Badosa of Spain topped American Louisa Chirico 6-0, 6-3.
Transylvania Open
Second-seeded Anhelina Kalinina of Ukraine defeated Belgium's Ysaline Bonaventure 6-3, 6-2 to advance to the quarterfinals at Cluj-Napoca, Romania.
No. 4 Anastasia Potapova of Russia defeated Viktorija Golubic of Switzerland 6-2, 6-3. No. 7 Xiyu Wang also advanced, needing three hours and 26 minutes to defeat Romanian Elena-Gabriela Ruse 6-1, 6-7 (8), 7-6 (6).
