No.2 seed Karolina Pliskova continues her title defence against Alison Riske, over whom she holds a commanding 7-1 head-to-head lead (4-1 on hard courts), including the last six matches stretching back to 2013; Riske's sole win in the series was in the first round of the 2012 ITF W50 event in The Bronx, which she took 3-6, 7-6(5), 6-2. However, their contests have been tighter than the head-to-head would imply: their last five matches have featured six tiebreaks, and two of the last three went to a deciding set - including the 2018 Tokyo quarterfinals, when Pliskova needed to save two match points before surviving 6-1, 6-7(5), 7-6(4). Since then, the Czech also posted a 7-6(3), 7-6(5) win in the third round of Dubai last year and a 6-4, 6-7(4), 6-2 victory in the second round of Toronto in August.
Danielle Collins, who has lost just three games in two rounds this week, defeated No.8 seed Madison Keys 6-3, 7-6(1) in their only previous meeting, in the second round of Indian Wells 2018 - the first of nine Top 20 victories that Collins has scored so far in her career.
No.3 seed Naomi Osaka's two previous encounters against No.6 seed Kiki Bertens have taken place with both players at very different stages of their careers. Osaka won the only completed match 6-4, 6-2 in the first round of Acapulco 2016 when she was ranked World No.114 to Bertens' World No.106; two years later, they would meet again as Top 10 players in the round robin of the 2018 WTA Finals, with Osaka retiring after losing the first set 6-3.
Qualifier Jennifer Brady, who notched up the first Top 10 win of her career at the eighth attempt in a second-round upset of World No.1 Ashleigh Barty, now faces No.5 seed and 2011 champion Petra Kvitova for the second time. Their first encounter, a 7-5, 1-6, 6-3 win for Kvitova in the third round of Dubai last year, was the first time Brady took a set off a Top 10 player.
ORDER OF PLAY (starts 11am)
PAT RAFTER ARENA
Danielle COLLINS (USA) vs  Madison KEYS (USA)
[Q] Jennifer BRADY (USA) vs  Petra KVITOVA (CZE)
 Kiki BERTENS (NED) vs  Naomi OSAKA (JPN)
Not before 7pm
Alison RISKE (USA) vs  Karolina PLISKOVA (CZE)
 HSIEH Su-Wei (TPE) / Barbora STRYCOVA (CZE) vs Lucie HRADECKA (CZE) / Andreja KLEPAC (SLO)