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.