Caroline Wozniacki lost the opening match of her WTA Finals defence with a 6-2 6-4 upset to seventh seed Karolina Pliskova.
The Czech needed just 32 minutes to wrap up the first set, coming from a game two break points down she reeled off five games in succession on her way to closing it out 6-2.
Wozniacki survived a tight service game, saving two match points, to reduce her second-set deficit to 5-4 but could not prevent Pliskova serving for victory.
"They are always tough matches against Caroline," she said in her post-match interview.
"I felt pretty good from the baseline today and I thought she was a bit nervous at the start as it's always tough to defend your title."
Ukraine's Elina Svitolina claimed only her second win in nine meetings with number four seed Petra Kvitova, beating the Czech 6-3 6-3 to take the lead in the round-robin portion of the event.
"It's very nice to be back and I'm very happy with the match and very pleased with the performance," said Svitolina, who completed her victory in 88 minutes with four breaks of service.
"I had to really stay focused. She strikes the ball pretty hard so I had to react quickly and move my feet.
"You have to bring the best level you can bring because everyone is strong and from the beginning you have tough matches."
