ORDER OF PLAY
CENTRE COURT (starts at 1.30pm)
Yung-Jan CHAN (TPE) / Martina HINGIS (SUI)  vs Timea BABOS (HUN) / Andrea HLAVACKOVA (CZE) 
Not before 4pm
Karolina PLISKOVA (CZE)  vs Caroline WOZNIACKI (DEN) 
Not before 7.30pm
Venus WILLIAMS (USA)  vs Caroline GARCIA (FRA) 
Kiki BERTENS (NED) / Johanna LARSSON (SWE) vs Ekaterina MAKAROVA (RUS) / Elena VESNINA (RUS) 
Caroline Wozniacki and Karolina Pliskova have played each other eight times to date, including five encounters in 2017. Wozniacki leads the head-to-head 5-3 overall, and 5-2 on hard courts. However Pliskova, who had never beaten the Dane before this year, leads 3-2 in the past 12 months - including their most recent meeting in the Cincinnati quarterfinals.
All five of Wozniacki's victories over Pliskova have been over three sets; all three of Pliskova's defeats of Wozniacki have been straight-sets wins. The two former World No.1s have never played each other indoors before.
Venus Williams and Caroline Garcia have not played each other since they squared off in successive weeks in the 2014 Asian swing - their only meetings to date. Garcia won the first of those matches 6-4, 4-6, 7-6(6) in the first round of Wuhan; Williams got her revenge in the second round of Beijing 6-4, 6-3.
Venus Williams will be playing her fifth WTA Finals semifinal, and third since the introduction of the round robin format. She lost her first two semifinals - in 1999 to Martina Hingis and in 2002 to Kim Clijsters (via retirement). However, she is 2-0 at this stage following a round robin, beating Jelena Jankovic in both 2008 (before going on to lift the trophy with a final victory over Vera Zvonareva) and 2009 (before falling to Serena Williams in the final).
Caroline Wozniacki has progressed to the semifinals at the WTA Finals for the fourth time in five appearances. She is 1-2 at this stage so far, losing to Serena Williams in 2009 and 2014 (the former by retirement, the latter in a third-set tiebreak) but beating Vera Zvonareva in 2010 (before falling to Kim Clijsters in the final).
Both Karolina Pliskova and Caroline Garcia are making their debuts in the final four of the WTA Finals. Garcia's presence marks the fourth straight year that a WTA Finals debutante has made it to the semifinal stage following Simona Halep (runner-up in 2014), Garbiñe Muguruza (semifinalist in 2015) and Dominika Cibulkova (champion in 2016).
Venus Williams is the only semifinalist yet to win a title in 2017 - but of the last four, only Karolina Pliskova had lifted a trophy prior to the Asian swing. Pliskova's total of three titles is the most of all the remaining players, followed by Garcia with two and Wozniacki with one.
Venus Williams is also the only semifinalist to have reached a Grand Slam final in 2017, having been runner-up at the Australian Open and Wimbledon. Each of the other three semifinalists' best Slam showing in 2017 came at Roland Garros, where Karolina Pliskova was a semifinalist and Caroline Garcia and Caroline Wozniacki were quarterfinalists.