Serena Williams starts a strong favourite to win a second successive WTA Championships as the end-of-season finale begins in Istanbul on Tuesday.
The world number one has only lost four matches and won 10 titles this season, more than any other year of her career.
She will face Petra Kvitova, Agnieszka Radwanska and Angelique Kerber in the group stages in Turkey.
"It's a tough group," Williams said. "That's the thing about this event, you have to be ready from the first match."
Eight of the top nine in the world rankings - world number three Maria Sharapova is injured - are split into two groups of four, with the top two in each qualifying for the semi-finals.
Australian Open champion Victoria Azarenka, who has beaten Williams twice this year, heads the other group, which includes China's Li Na, Sara Errani of Italy and Serb Jelena Jankovic.
Three group matches will be played on each of the first four days.
Tuesday's opening day sees Belarusian Azarenka start against Errani (15:00 BST) before Williams faces Germany's Kerber (17:00 BST) and Radwanska of Poland plays the Czech Republic's Kvitova (19:00 BST).
Williams, 32, won the French Open and US Open this year to take her total of Grand Slam titles to 17, one short of Martina Navratilova and Chris Evert and five behind Steffi Graf's Open era record of 22.
"I never thought I would have one of my best years this year, and now there is an opportunity to do even better," she added.
"I'm still looking to improve and...I have a lot of room for improvement. I am interested for next year to take my game to a new level, which I think I can do.
"It's going to be even tough for me to get to 18 [ Grand Slams]. Obviously that's my goal, but I take it one match at a time."
The American is aiming to become the first player to defend a WTA Championships crown since Justine Henin in 2007. If she achieves the feat, it would be her fourth title overall.
While Kvitova, who won in 2011, is a dangerous opponent on a fast indoor surface, neither she nor Radwanska has ever beaten Williams, while Kerber can boast one victory from their three previous meetings.
Jankovic is the only player to qualify this year who was not in the field 12 months ago. The Serb is back among the top eight of the season for the first time since 2010.