No.4 seed and Wimbledon champion Simona Halep faces qualifier Jennifer Brady, who has three Top 20 wins this season - all of them Caroline Garcia in Hua Hin, Dubai and Indian Wells - but has taken just one set from a Top 10 player in five meetings, her 7-5, 1-6, 6-3 loss to Petra Kvitova in Dubai this year.
Both of No.2 seed Naomi Osaka's prior encounters with Tatjana Maria were qualifying matches in 2016, and the results were split 1-1: Osaka won 7-5, 6-7(6), 6-3 in Hobart and Maria got her revenge 6-2, 6-1 in Beijing. In both cases, the player ranked outside the Top 100 at the time beat the player ranked inside it. Maria has four Top 10 wins to her credit in her career, but would be bidding for her first Top 3 scalp.
No.6 seed and Wimbledon semifinalist Elina Svitolina has won both of her meetings with Katerina Siniakova - but both went the distance, with the Ukrainian squeezing past 6-0, 6-7(5), 6-3 in the first round of the 2017 US Open and 4-6, 6-2, 6-1 in the second round of the Australian Open the following year. Siniakova is seeking her second Top 10 victory of 2019 following her stunning upset of Naomi Osaka at Roland Garros, and her sixth Top 10 win overall.
Three-time champion Serena Williams and two-time champion Simona Halep are the only players who have won the Rogers Cup multiple times this decade. All three of Williams's titles have come in Toronto (2001, 2011 and 2013), while both of Halep's have come in Montréal (2016, 2018). Williams's first opponent Elise Mertens is seeking her first Top 10 victory since beating Halep in the Doha final in February.
No.11 seed and 2015 champion Belinda Bencic trails Julia Goerges 1-2 in their head-to-head, having lost to the German in both of their previous hard court matches - outdoors in the first round of the 2015 Australian Open 6-2, 6-1 and indoors in last year's Luxembourg final 6-4, 7-5. Bencic's sole win in the series was a 3-6, 6-3, 7-5 epic on Goerges' home turf of Stuttgart in 2015.
No.3 seed Karolina Pliskova dominates the head-to-head against qualifier Alison Riske, leading 6-1 - including the last five matches stretching back to 2013. However, the pair have not played on North American hard courts since two meetings in 2012 at ITF level, with Pliskova winning their first clash in Nassau 7-5, 6-2 and Riske taking her only win in the series 3-6, 7-6(5), 6-2 in The Bronx, and in their penultimate clash Pliskova needed to save two match points before overcoming the American 6-1, 6-7(5), 7-6(4) in last year's Tokyo quarterfinals. Their most recent meeting was in the third round of Dubai this February, with the Czech former World No.1 taking victory 7-6(3), 7-6(5) - the fourth of their matches to feature at least one tiebreak. Overall, Pliskova has won four tiebreaks to Riske's two.
19-year-old three-time WTA titlist Dayana Yastremska is one of three teenagers to have reached the second round. The Ukrainian faces three-time semifinalist Victoria Azarenka for the first time today; she is 2-3 against players who have been ranked World No.1, with both of her wins coming against Garbiñe Muguruza.
Former Roland Garros champion Jelena Ostapenko takes on Anastasia Pavlyuchenkova for the first time on outdoor hard courts today. Ostapenko leads the overall head-to-head 3-1, having won twice on clay (in a 2018 Fed Cup World Group II play-off and in the first round of Madrid this year 6-2, 6-1) and once on grass (at Birmingham in 2016). Pavlyuchenkova's sole victory came in the second round of St Petersburg this February, the Russian winning 6-4, 6-4.
Both of No.16 seed Anett Kontaveit's meetings with Carla Suárez Navarro took place on Australian hard courts in 2018. Kontaveit routed Suárez Navarro 6-0, 6-3 in the first round of Brisbane - but three weeks later, the Spaniard got her revenge, coming back from 1-4 down in the third set to triumph in a 4-6, 6-4, 8-6 epic in the fourth round of the Australian Open to make her sixth major quarterfinal out of seven to date.
18-year-old qualifier Iga Swiatek is the youngest player remaining in the draw, and will bid for her second career Top 20 win in just her third match against a player ranked inside that echelon against No.15 seed Caroline Wozniacki.
Wildcard Svetlana Kuznetsova's last match against Donna Vekic was a classic final in Washington last year in which the Russian saved four match points to capture her 18th career title 4-6, 7-6(7), 6-2. Overall, Kuznetsova leads the series 2-1: Vekic won their first encounter 7-6(5), 7-5 in the first round of Miami in 2014, but Kuznetsova won their next 7-5, 6-4 in the second round of Tianjin in 2016.
Yesterday, Donna Vekic and Alison Riske became the fourth and fifth players to win from match point down twice in 2019 (joining Viktoria Kuzmova, Karolina Pliskova and Tamara Zidansek). Riske's defeat of Maria Sakkari, who had beaten Aryna Sabalenka from four match points down in San Jose last week, meant that the Greek became the third player this season to lose from match point up in her next tournament after winning from match point down (joining Jelena Ostapenko in Charleston and Bogota and, ironically, Riske herself in 's-Hertogenbosch and Mallorca).
ORDER OF PLAY (all courts start 11am)
 Karolina PLISKOVA (CZE) vs [Q] Alison RISKE (USA)
[Q] Jennifer BRADY (USA) vs  Simona HALEP (ROU)
Daria KASATKINA (RUS) vs Bianca ANDREESCU (CAN)
Not before 7pm
 Serena WILLIAMS (USA) vs Elise MERTENS (BEL)
 Caroline WOZNIACKI (DEN) vs [Q] Iga SWIATEK (POL)
Victoria AZARENKA (BLR) vs Dayana YASTREMSKA (UKR)
Katerina SINIAKOVA (CZE) vs  Elina SVITOLINA (UKR)
Not before 3pm
Jelena OSTAPENKO (LAT) vs Anastasia PAVLYUCHENKOVA (RUS)
Not before 5pm
[Q] Tatjana MARIA (GER) vs  Naomi OSAKA (JPN)
Not before 7pm
Donna VEKIC (CRO) vs [WC] Svetlana KUZNETSOVA (RUS)
Carla SUÁREZ NAVARRO (ESP) vs  Anett KONTAVEIT (EST)
 Belinda BENCIC (SUI) vs Julia GOERGES (GER)
[Q] Ekaterina ALEXANDROVA (RUS) vs [LL] ZHANG Shuai (CHN)
 Nicole MELICHAR (USA) / Kveta PESCHKE (CZE) vs Jennifer BRADY (USA) / Katarina SREBOTNIK (SLO)
Miyu KATO (JPN) / Sabrina SANTAMARIA (USA) vs  Gabriela DABROWSKI (CAN) / XU Yifan (CHN)
Shuko AOYAMA (JPN) / Kaitlyn CHRISTIAN (USA) vs  Anna-Lena GROENEFELD (GER) / Demi SCHUURS (NED)
 Kirsten FLIPKENS (BEL) / HSIEH Su-Wei (TPE) vs Eri HOZUMI (JPN) / Makoto NINOMIYA (JPN)
Nadiia KICHENOK (UKR) / Abigail SPEARS (USA) vs  CHAN Hao-Ching (TPE) / Latisha CHAN (TPE)
 Barbora KREJCIKOVA (CZE) / Katerina SINIAKOVA (CZE) vs Raluca OLARU (ROU) / YANG Zhaoxuan (CHN)