No.5 seed Simona Halep leads the head-to-head against No.2 seed Karolina Pliskova 7-3. Their only previous indoor hard court encounters have taken place in Fed Cup first-round ties, with each scoring one victory on the other's home turf: Pliskova won 6-7(5), 6-4, 6-2 in 2016 in Cluj-Napoca, paving the way for the Czech Republic to take the tie 3-2, but in 2019 Halep scored a 6-4, 5-7, 6-4 triumph en route to Romania's 3-2 revenge. However, their most recent meeting saw Pliskova defeat Halep 7-5, 6-1 in the Miami semifinals. Despite this being the third WTA Finals that both have contested together, today will mark their first clash at the year-end finale.
No.8 seed Elina Svitolina and alternate Sofia Kenin have split four previous meetings 2-2, all this year on outdoor hard courts. Both of Svitolina's victories came over three sets, 3-6, 7-6(5), 6-4 in the second round of Indian Wells and 6-3, 6-7(8), 6-3 in the third round of Beijing; in between, Kenin took two straight-sets wins during the US Open series, 7-6(2), 6-4 in the Toronto quarterfinals and 6-3, 7-6(3) a week later in the third round of Cincinnati.
Svitolina's place at the top of the Purple Group is assured, and she will face No.7 seed Belinda Bencic in Saturday's semifinals regardless of today's results. Halep and Pliskova are fighting for second place in the group, and the winner will face No.1 seed Ashleigh Barty in the last four.
Kenin will substitute for No.4 seed Bianca Andreescu, who suffered a knee injury during her match against Pliskova, marking the first time since 2009 that two alternates have competed at the WTA Finals. In 2009, first alternate Vera Zvonareva replaced Dinara Safina and second alternate Agnieszka Radwanska substituted for Zvonareva.
ORDER OF PLAY (starts 4pm)
 Elise MERTENS (BEL) / Aryna SABALENKA (BLR) vs  Timea BABOS (HUN) / Kristina MLADENOVIC (FRA)
Not before 6.30pm
[Alt] Sofia KENIN (USA) vs  Elina SVITOLINA (UKR)
Not before 8pm
 Karolina PLISKOVA (CZE) vs  Simona HALEP (ROU)
 CHAN Hao-Ching (TPE) / Latisha CHAN (TPE) vs  Anna-Lena GROENEFELD (GER) / Demi SCHUURS (NED)