Defending champion Angelique Kerber takes on 2015 winner Petra Kvitova in a blockbuster quarterfinal. The pair have been facing off since 2012, and their head-to-head is all square at 6-6 - as is their outdoor hard court head-to-head at 2-2. Kvitova and Kerber faced each other twice last season - in back-to-back weeks at the same venue, with the Czech winning their Fed Cup semifinal rubber 6-2, 6-2 and Kerber taking their Stuttgart first-round tilt on the same court 6-3, 6-2. Seven of their 12 contests to date have gone to a deciding set.
Kiki Bertens leads her head-to-head against Yulia Putintseva 4-1, including their only previous hard court meeting - their first encounter in Cincinnati qualifying back in 2012. The Kazakh, who is bidding for her second career Premier semifinal following St Petersburg 2017, captured her sole win over Bertens 6-4, 5-7, 7-5 in the second round of Nurnberg in 2015.
Timea Bacsinszky was ranked World No.745 as recently as August, having gone winless for a 14-month spell that included hand surgery and a difficult initial comeback. The Swiss player is now ranked No.192 and will compete in the first Premier-level quarterfinal of her comeback against Aliaksandra Sasnovich. The Belarusian was the victor in the pair's only previous meeting, a 6-2, 7-6(2) win in the 2017 Fed Cup semifinals.
Ashleigh Barty is on a five-match winning streak at WTA level, having come back to win the Zhuhai Elite Trophy at the end of 2018 after losing her first round-robin rubber of the event to Aryna Sabalenka. All five of Barty's wins have been over Top 25 opposition, culminating in her first victory over a World No.1 yesterday against Simona Halep. Last week, the Australian also compiled a 2-1 record in Hopman Cup, beating Alizé Cornet and Garbiñe Muguruza before losing to Angelique Kerber.
Two-time defending Hobart champion Elise Mertens opted not to go for a hat-trick of titles in Tasmania this week, and her decision to compete at Premier level has paid off in a quarterfinal on her Sydney debut. Mertens will face Barty for the first time to make the last four.
ORDER OF PLAY (all courts start 1pm)
KEN ROSEWALL ARENA
After 1 ATP match, not before 2.30pm
Ashleigh BARTY (AUS) vs Elise MERTENS (BEL) 
After 1 ATP match, not before 7pm
Petra KVITOVA (CZE)  vs Angelique KERBER (GER) 
SHOW COURT 1
[Q] Aliaksandra SASNOVICH (BLR) vs Timea BACSINSZKY (SUI)
After 1 ATP match
[Q] Yulia PUTINTSEVA (KAZ) vs Kiki BERTENS (NED) 
Andreja KLEPAC (SLO) / María José MARTÍNEZ SÁNCHEZ (ESP)  vs Eri HOZUMI (JPN) / Alicja ROSOLSKA (POL)
Gabriela DABROWSKI (CAN) / XU Yifan (CHN)  vs Aleksandra KRUNIC (SRB) / Katerina SINIAKOVA (CZE)
After suitable rest
Gabriela DABROWSKI (CAN) / XU Yifan (CHN)  or Aleksandra KRUNIC (SRB) / Katerina SINIAKOVA (CZE) vs Miyu KATO (JPN) / Makoto NINOMIYA (JPN)