ORDER OF PLAY
MANOLO SANTANA -- 12:00 noon start
 Simona HALEP (ROU) vs.  Ashleigh BARTY (AUS)
Following 3 ATP matches, not before 9:30 p.m.
 Kiki BERTENS (NED) vs.  Petra KVITOVA (CZE)
ARANTXA SANCHEZ -- 12:00 noon start
Following 1 ATP match, not before 2:00 p.m.
 Naomi OSAKA (JPN) vs. Belinda BENCIC (SUI)
Not before 4:00 p.m.
 Sloane STEPHENS (USA) vs. Petra MARTIC (CRO)
COURT 5 -- 12:00 noon start
Irina BARA (ROU) / Mihaela BUZARNESCU (ROU) vs. [WC] Jelena OSTAPENKO (LAT) / Vera ZVONAREVA (RUS)
 Anna-Lena GROENEFELD (GER) / Demi SCHUURS (NED) vs. Veronika KUDERMETOVA (RUS) / Galina VOSKOBOEVA (KAZ)
 Barbora KREJCIKOVA (CZE) / Katerina SINIAKOVA (CZE) vs.  Gabriela DABROWSKI (CAN) / XU Yifan (CHN)
 HSIEH Su-Wei (TPE) / Barbora STRYCOVA (CZE) vs.  Samantha STOSUR (AUS) / ZHANG Shuai (CHN)
Three-time champion Petra Kvitova and Kiki Bertens will face off in a rematch of last year's Mutua Madrid Open final, which Kvitova won in three sets. Kvitova leads their head-to-head by 3-2 overall, and by 2-0 on clay, including a tough three-set win in the Stuttgart semifinals last month.
Simona Halep leads her head-to-head with Ashleigh Barty 2-1. Halep won both of their meetings in 2018 in straight sets, in the Montreal semifinals and the Cincinnati round of 16. However, Barty won their most recent match, in the second round of Sydney this year.
Halep improved her career win-loss record at Madrid to 23-6 after her third-round win over Viktoria Kuzmova. The Romanian, who won the Madrid title in 2016 and 2017, notched a double-bagel victory over Kuzmova -- her first match-win without losing a game since 2016.
World No.1 Naomi Osaka and Belinda Bencic have split their two prior encounters. Bencic won their only meeting at WTA-level, which was a 6-3, 6-1 upset at the BNP Paribas Open in March, ending Osaka's title defense in Indian Wells.
Sloane Stephens has won both of her previous meetings with Petra Martic, but this will be their first clash on clay. Stephens eked out a win over Martic in the third round of the Australian Open earlier this year, in two tiebreak sets.