No.1 seed and 2018 semifinalist Ashleigh Barty takes on Sofia Kenin for the third time in the past 14 months, having already defeated the American in Charleston last year, in a Fed Cup first-round tie in February and in the fourth round of Roland Garros en route to her maiden Grand Slam title - the last of those, a 6-3, 3-6, 6-0 win, being the only time Barty has dropped a set in the series so far. Kenin has three Top 10 wins under her belt so far, all in the past 14 months, but has yet to defeat an opponent ranked in the Top 5.
Venus Williams leads the head-to-head against Carla Suárez Navarro 7-3, including 4-1 on North American hard courts. Between the 2009 Australian Open and Miami 2015, Suárez Navarro won three of their first five meetings - but starting at Wuhan 2015, Williams has won the last five in a row for the loss of just one set, including a 7-6(4), 6-1 victory this March in the first round of Miami. The pair have played once before in Canada: Williams won their 2014 Montréal quarterfinal 4-6, 6-2, 6-3.
Washington runner-up Camila Giorgi is 2-0 against Victoria Azarenka, having beaten the Belarusian 4-6, 6-3, 7-5 in the first round of Eastbourne in 2014 and then via retirement in last year's Tokyo quarterfinals. Former World No.1 Azarenka is a three-time semifinalist in Canada, having made the last four in 2008, 2010 and 2011 - but is yet to reach the final here.
No.10 seed Anastasija Sevastova is 3-1 against Wimbledon quarterfinalist Zhang Shuai. Lucky loser Zhang's sole win in the series came 6-4, 7-5 in Estoril qualifying in 2012, but since then the Latvian has not dropped a set, including a 6-0, 7-6(10) win in the round robin stages of last year's Zhuhai Elite Trophy - their most recent encounter and only time they have played on hard courts before.
An all-Canadian derby finds Indian Wells champion Bianca Andreescu taking on former World No.5 Eugenie Bouchard. Andreescu won their only previous encounter 6-2, 6-0 in the Newport Beach 125K quarterfinals in January, but the 19-year-old has completed just one match since retiring in the fourth round of Miami in March due to a shoulder injury.
No.14 seed Madison Keys and Donna Vekic will clash for the first time off clay today. 2016 finalist Keys has not dropped a set in two prior matches with the Croat, winning 6-3, 6-2 in the first round of Roland Garros 2016 and 7-6(2), 7-6(0) in the second round of Rome 2018.
San Jose champion Zheng Saisai takes on qualifier Tatjana Maria for the first time in a battle of spin and slice. Only three players in the Open Era have swept both the Californian and Canadian titles in the same year: Arantxa Sánchez-Vicario in 1994 (Oakland and Montréal), Kim Clijsters in 2005 (Stanford and Toronto) and Serena Williams in 2011 (Stanford and Toronto).
Australian Open quarterfinalist Anastasia Pavlyuchenkova will bid to snap a six-match losing streak dating back to Stuttgart in May in a first tilt against No.9 seed and San Jose runner-up Aryna Sabalenka.
ORDER OF PLAY (all courts start 11am)
Carla SUÁREZ NAVARRO (ESP) vs Venus WILLIAMS (USA)
Victoria AZARENKA (BLR) vs Camila GIORGI (ITA)
 Ashleigh BARTY (AUS) vs Sofia KENIN (USA)
Not before 7pm
[WC] Eugenie BOUCHARD (CAN) vs Bianca ANDREESCU (CAN)
 Sloane STEPHENS (USA) vs [Q] Marie BOUZKOVA (CZE)
Danielle COLLINS (USA) vs Katerina SINIAKOVA (CZE)
Maria SAKKARI (GRE) vs [Q] Alison RISKE (USA)
 Madison KEYS (USA) vs Donna VEKIC (CRO)
Not before 5pm
Anastasia PAVLYUCHENKOVA (RUS) vs  Aryna SABALENKA (BLR)
[Q] Francesca DI LORENZO (USA) vs  Kiki BERTENS (NED)
[LL] ZHANG Shuai (CHN) vs  Anastasija SEVASTOVA (LAT)
 Nicole MELICHAR (USA) / Kveta PESCHKE (CZE) vs [WC] Leylah FERNANDEZ (CAN) / Simona HALEP (ROU)
[Q] Iga SWIATEK (POL) vs [Q] Ajla TOMLJANOVIC (AUS)
Raluca OLARU (ROU) / YANG Zhaoxuan (CHN) vs Sofia KENIN (USA) / Anastasija SEVASTOVA (LAT)
[Q] Misaki DOI (JPN) vs [Q] Ekaterina ALEXANDROVA (RUS)
ZHENG Saisai (CHN) vs Tatjana MARIA (GER)
PENG Shuai (CHN) / ZHANG Shuai (CHN) vs  CHAN Hao-Ching (TPE) / Latisha CHAN (TPE)
Darija JURAK (CRO) / María José MARTÍNEZ SÁNCHEZ (ESP) vs DUAN Yingying (CHN) / ZHENG Saisai (CHN)
Julia GOERGES (GER) / Karolina PLISKOVA (CZE) vs Lyudmyla KICHENOK (UKR) / Jelena OSTAPENKO (LAT)
 Lucie HRADECKA (CZE) / Andreja KLEPAC (SLO) vs Timea BABOS (HUN) / Anett KONTAVEIT (EST)
Eri HOZUMI (JPN) / Makoto NINOMIYA (JPN) vs Anastasia POTAPOVA (RUS) / Galina VOSKOBOEVA (KAZ)
Jennifer BRADY (USA) / Katarina SREBOTNIK (SLO) vs Desirae KRAWCZYK (USA) / Alicja ROSOLSKA (POL)
Alexa GUARACHI (CHI) / Asia MUHAMMAD (USA) vs Shuko AOYAMA (JPN) / Kaitlyn CHRISTIAN (USA)