No.9 seed Belinda Bencic's tantalizing first-round clash with Hsieh Su-Wei is a rematch of the 2017 Hua Hin 125K final, won by Bencic 6-3, 6-4 as part of a 15-match winning streak as she returned from injury that year. It was the Swiss player's second win in as many meetings with Hsieh, having also triumphed 6-4, 4-6, 6-2 in the 2014 Tianjin quarterfinals en route to her maiden WTA final.
No.2 seed Karolina Pliskova leads former Roland Garros champion Jelena Ostapenko 3-1 - although two of those wins have been epic tussles. Pliskova took their first encounter in the second round of Cincinnati 2016 in straight sets, but had to battle to a 4-6, 6-0, 10-8 victory in the third round of the 2017 Australian Open and a 5-7, 7-5, 6-4 win in the 2018 Stuttgart quarterfinals. In between, Ostapenko would seize their 2017 WTA Finals round-robin encounter 6-3, 6-1.
Daria Kasatkina, who has compiled just a 9-19 win-loss record in 2019, lost her only previous match against wildcard Peng Shuai 6-0, 7-6(5) in the first round of the 2017 Australian Open.
No.8 seed Kiki Bertens will reprise February's St Petersburg final against Donna Vekic, which Bertens took 7-6(2), 6-4. Overall, Vekic leads the head-to-head 3-2, having won 6-3, 1-6, 6-3 in the first round of Tashkent in 2015, 7-6(3), 6-3 in the second round of the 2016 San Antonio 125K event and 7-6(5), 1-6, 7-5 in the second round of Brisbane this January - saving two match points in the last of those. Bertens' other win over the Croat came in 2016 Miami qualifying.
2015 champion Garbiñe Muguruza, who faces No.15 seed Sofia Kenin for the first time, is one of four former winners in this year's main draw alongside 2006 and 2009 champion Svetlana Kuznetsova, 2010 and 2018 champion Caroline Wozniacki and 2017 champion Caroline Garcia.
San Jose champion Zheng Saisai defeated Monica Puig 6-4, 6-2 in their only prior encounter, in the first round of the 2016 US Open.
ORDER OF PLAY (all courts start 11am except Diamond)
DIAMOND (starts 12.30pm)
ZHENG Saisai (CHN) vs Monica PUIG (PUR)
Donna VEKIC (CRO) vs  Kiki BERTENS (NED)
[WC] Jelena OSTAPENKO (LAT) vs  Karolina PLISKOVA (CZE)
Not before 7.30pm
Daria KASATKINA (RUS) vs [WC] PENG Shuai (CHN)
Garbiñe MUGURUZA (ESP) vs  Sofia KENIN (USA)
Qualifying - YUAN Yue (CHN) vs  Bernarda PERA (USA)
Not before 12.30pm
 Belinda BENCIC (SUI) vs HSIEH Su-Wei (TPE)
Katerina SINIAKOVA (CZE) vs [WC] WANG Xiyu (CHN)
Polona HERCOG (SLO) vs Julia GOERGES (GER)
Alicja ROSOLSKA (POL) / Storm SANDERS (AUS) vs  Samantha STOSUR (AUS) / ZHANG Shuai (CHN)
After 1 ATP match, not before 12 noon
Qualifying -  Lauren DAVIS (USA) vs  Anastasia POTAPOVA (RUS)
Qualifying -  Tamara ZIDANSEK (SLO) vs Christina MCHALE (USA)
Qualifying -  Rebecca PETERSON (SWE) vs  Misaki DOI (JPN)
Qualifying -  Magda LINETTE (POL) vs Heather WATSON (GBR)
Qualifying -  Marie BOUZKOVA (CZE) vs  Andrea PETKOVIC (GER)
Qualifying -  Jennifer BRADY (USA) vs  Kateryna KOZLOVA (UKR)
Qualifying -  Anna BLINKOVA (RUS) vs  Zarina DIYAS (KAZ)