VOLVO CAR STADIUM -- 11:00 a.m. start
Taylor TOWNSEND (USA) vs.  Belinda BENCIC (SUI)
 Maria SAKKARI (GRE) vs.  Kiki BERTENS (NED)
 Caroline WOZNIACKI (DEN) vs.  Mihaela BUZARNESCU (ROU)
 Jelena OSTAPENKO (LAT) vs.  Madison KEYS (USA)
Not before 7:00 p.m.
 Sloane STEPHENS (USA) vs.  Ajla TOMLJANOVIC (AUS)
 Anna-Lena GROENEFELD (GER) / Alicja ROSOLSKA (POL) vs. Shuko AOYAMA (JPN) / Lidziya MAROZAVA (BLR)
ALTHEA GIBSON CLUB COURT -- 11:00 a.m. start
 Petra MARTIC (CRO) vs. Jessica PEGULA (USA)
 Aryna SABALENKA (BLR) vs. Monica PUIG (PUR)
 Danielle COLLINS (USA) vs. Kaia KANEPI (EST)
 Nicole MELICHAR (USA) / Kveta PESCHKE (CZE) vs. Anna-Lena FRIEDSAM (GER) / Sara SORRIBES TORMO (ESP)
Two former Charleston finalists face off in the third round when 2015 runner-up Madison Keys meets 2017 runner-up Jelena Ostapenko. Keys won their only prior encounter, a three-set quarterfinal on the grass of Birmingham in 2016.
Taylor Townsend and Belinda Bencic have yet to have a professional meeting, however, they were junior rivals who contested the Wimbledon girls' singles final in 2013. Bencic won that match for the second of her two Grand Slam junior singles titles of that year (she also triumphed at the French Open).
Defending champion Kiki Bertens and Maria Sakkari have played five times, all of which came in the last three years. Bertens has a slender 3-2 lead in their head-to-head, having won their most recent match in the semifinals of Seoul last season. They are 1-1 on clay.
2011 Charleston champion Caroline Wozniacki has won her two previous matches against Mihaela Buzarnescu, which each came in the first round of back-to-back Grand Slam events, the 2017 US Open (Buzarnescu's Grand Slam main draw debut) and the 2018 Australian Open. Wozniacki went on to win her first Grand Slam title at the latter event.
Read more: Wozniacki makes winning return to Charleston
2016 Charleston champion Sloane Stephens has a commanding 4-0 head-to-head lead against Ajla Tomljanovic. Their last meeting came in the second round of last year's Miami Open. Stephens ended up claiming the title at that event, which is her most recent singles championship.
Petra Martic has won two of her three clashes with Jessica Pegula, but Pegula won their last encounter in the quarterfinals of Quebec City last year, en route to her first-ever WTA singles final.