Agnieszka Radwanska got her WTA Championships campaign off to a great start with a 6-3 6-2 victory over defending champion Petra Kvitova in Istanbul.
The title holder came into the match on a 25-match winning streak indoors but she produced an erratic performance, losing to Poland's Radwanska for the first time in four matches.
Wimbledon finalist Radwanska has enjoyed a successful year and the Pole raced into a 3-0 lead, as the round-robin stage of this season-ending event got underway.
But Kvitova then saved a quartet of break points in a lengthy fourth game before winning it to get off the mark and, suitably encouraged, she added the next two games to level at 3-3.
But the Czech then lost her way again and the more consistent Radwanska reeled off three games in a row to take the first set.
And Kvitova then endured a nightmare on serve in the second set, broken on all four occasions as Radwanska made a winning start in the White Group, which also includes Maria Sharapova and Sara Errani.
"I'm just very happy about my performance," Radwanska said after the match.
"The first match is always tough and adjusting to a new surface is never easy, but I was playing really well from the beginning to the end of the match.
"It was still a little bit up and down today. There were a lot of breaks of serve in the match - the serve should be a big weapon, and it wasn't in this match.
"But I just stayed focused and I'm happy I closed this match out in two sets."
Kvitova said: "I was not very comfortable on court and I didn't feel pretty well."