Istanbul, Turkey , 24 October 2011 | AP

US Open champion Samantha Stosur and top-ranked Caroline Wozniacki head two round-robin groups at the WTA Championships this week as Turkey’s biggest city hosts the event for the first time.

With Stosur in the white group are three-time major winner Maria Sharapova, Victoria Azarenka and French Open champion Li Na.

Wozniacki is joined in the red group by Wimbledon champion Petra Kvitova, Vera Zvonareva and Agnieszka Radwanska.

Each player meets the other three players in their group, and the top two in each pool move into the semi-finals.

The final is on Sunday.

“I think we all know what to expect going into the groups,” Sharapova said.

“It’s the top eight girls of the year. You’re going to get a tough group either way and a tough opponent. I think it’s just a matter of being ready from the first round on.”

Thirteen-time grand slam singles champion Serena Williams failed to qualify for the tournament, as did her sister Venus.

Wozniacki, who has won six titles this year but has yet to win a major, said she was preparing for tough matches.

“I’m going in there just hoping to play my best tennis and we’ll see what happens,” she said.

Play begins on Tuesday (0100 AEDT on Wednesday) at the Sinan Erdem Arena in Istanbul, which will host the $US4.9 million ($A4.8 million) event until 2013.

Qatar hosted the tournament for the last three years.