2016 champion Angelique Kerber dominates the head-to-head against Madison Keys 6-1, and has never lost a set to the American in five hard court meetings. Keys' sole victory came over three sets in the 2014 Eastbourne final. The pair have played once before in Melbourne, in the third round in 2013 - also their only prior Slam meeting - which Kerber took 6-2, 7-5.
Simona Halep leads the head-to-head against Karolina Pliskova 5-1. The Czech's only win was a 6-7(5), 6-4, 6-2 first-round Fed Cup rubber in 2016, on Halep's home soil in Cluj-Napoca. Aside from that match, the Romanian has won four hard court encounters in straight sets and one clay meeting in three sets, 6-4, 3-6, 6-3 in the Roland Garros semifinals last year (their most recent match and sole previous Slam encounter).
Simona Halep is bidding to become the first Romanian semifinalist at the Australian Open, while Pliskova would be the first Czech since Petra Kvitova in 2012. Both have lost the previous Melbourne quarterfinals to opponents outside the Top 10: Pliskova to No.79 Mirjana Lucic-Baroni last year, and Halep to No.24 Dominika Cibulkova in 2014 and No.11 Ekaterina Makarova in 2015. The Romanian is yet to win a set at this stage, losing 6-3, 6-0 to Cibulkova and 6-4, 6-0 to Makarova.
The winner of the quarterfinal between Angelique Kerber and Madison Keys will return to the Top 10 - Kerber for the first time since August 2017, and Keys for the first time since April 2017. The loser will return to the Top 15.
While Angelique Kerber is the only former senior Australian Open champion remaining, Karolina Pliskova is a former junior champion here, beating Laura Robson 6-1, 7-6(5) in the 2010 girls' final. That year, Madison Keys lost in the second round of the girls' singles to Yana Buchina; in 2008, Simona Halep's semifinal run (losing to Jessica Moore) was her best junior showing in Melbourne; and in 2004, Kerber made the quarterfinals (losing to Jarmila Wolfe, née Gajdosova) for her best girls' result here.
Angelique Kerber and Simona Halep, champions in Sydney and Shenzhen this year respectively, are both on nine-match winning streaks at Tour level, with the German also winning four matches in Hopman Cup competition to start the year. Kerber is seeking to become the first player to win both Sydney and the Australian Open in the same year since Victoria Azarenka in 2012, while Halep would be the first to perform the Shenzhen-Melbourne double.
Remarkably, this year marks just Angelique Kerber's second quarterfinal in Melbourne, along with her 2016 title run. For Simona Halep, the Australian Open can be added to Wimbledon as the second Slam at which she has been a three-time quarterfinalist; and for Madison Keys, this tournament marks the first time that the 22-year-old has become a two-time quarterfinalist at a Slam.
Madison Keys has spent four hours and 10 minutes on court so far, the least time of any of the quarterfinalists; she has dropped just 19 games and, along with Elise Mertens, is the only player yet to lose a set. Conversely, Simona Halep's eight hours and 44 minutes are the longest any of the last eight have spent on court. Karolina Pliskova leads the ace count heading into the quarterfinals, having notched up 27 in Melbourne so far.
Angelique Kerber would notch up her 75th career Top 20 win with victory over Madison Keys today. Simona Halep is going for her second Top 10 win at the Australian Open (her previous one being a 6-4, 2-6 6-0 defeat of Jelena Jankovic in the fourth round in 2014) and sixth at a Slam overall. Meanwhile, a Karolina Pliskova win would be the 25-year-old's second triumph over a World No.1 (having defeated Serena Williams in the semifinals of the 2016 US Open) - but first Top 10 victory in Melbourne.
Elise Mertens has become the first Australian Open debutante to reach the semifinals since Johanna Konta in 2016. Joel Drucker breaks down the Belgian's game, and her time spent training at the Kim Clijsters Academy, for Tennis.com.
With her second consecutive deep Slam run, Madison Keys is putting her injury woes behind her - and, as she tells Justin Bergman at SFGate, playing with a greater sense of happiness.
ORDER OF PLAY
Seven-time Grand Slam champion Evonne Goolagong Cawley, one of Australia's greatest legends of the game, has been keeping a keen eye on this year's tournament - find out her thoughts on the state of the game, and which current player makes me want to pick up her own racket and play again.