Roger Federer suffered a surprise defeat in his opening match of the ATP Finals as Kei Nishikori raced to a 7-6 (7-4) 6-3 victory in London.
Kevin Anderson did not face a break point in his ATP Finals bow, beating Dominic Thiem in the first match at the O2 Arena.
Despite another battling performance from Sofia Kenin, United States were swept aside by Czech Republic in the Fed Cup final.
Evgeniya Rodina had no answer to Ekaterina Alexandrova as the world number 93 secured her second Open de Limoges crown.
Stefanos Tsitsipas overcame Alex de Minaur to win the Next Gen ATP Finals title.
Petra Kvitova's illness did not hinder Czech Republic as they raced into a 2-0 lead against the United States in the Fed Cup final.
Lucie Safarova will retire following next year's Australian Open due to health issues she has dealt with since 2015.
Novak Djokovic and Roger Federer have 11 ATP Finals titles between them and they are the two standout contenders in London.
Stefanos Tsitsipas and Alex de Minaur won their way through to the Next Gen ATP Final on Friday, defeating Andrey Rublev and Jaume Munar.
Coach Darren Cahill says he will support Simona Halep from the sidelines after opting to take a hiatus from his role in 2019.