Swiatek rallied from 4-1 down in the first set and squandered a 5-1 lead in the second set before prevailing in just over two hours.
8 seed Veronika Kudermetova of Russia, who defeated Kayla Day 6-3, 6-3 in second-round action at the WTA 500 event.
7 seed Liudmila Samsonova fell in straight sets to fellow Russian Ekaterina Alexandrova, 6-4, 6-2.
1 seed Ons Jabeur of Tunisia eased into the quarterfinals in Ningbo, China, with a 6-3, 6-2 win over Germany's Tamara Korpatsch.
4 Anna Blinkova of Russia 6-3, 3-6, 6-1.
Persons:
Iga Swiatek, Slovenia's Kaja Juvan, Brendan Mcdermid, Mai Hontama, Swiatek, Veronika Kudermetova, Kayla Day, Jessica Pegula, Spain's Cristina Bucsa, Liudmila Samsonova, Ekaterina Alexandrova, Germany's Tamara Korpatsch, Jabeur, Russia's Vera Zvonareva, Clara Tauson, Dane, Katerina Siniakova, Linda Fruhvirtova, Anna Blinkova, Lucia Bronzetti, Nadia Podoroska
Organizations:
Slovenia's Kaja Juvan REUTERS, Pan, WTA, Thomson
Locations:
Flushing Meadows , New York, United States, Poland, Tokyo, Russia, Ningbo, Jabeur, Tunisia, China, Czech, Romania, Italy, Argentina