Stosur, the US Open champion, was coming off her first ever win over Maria Sharapova late Tuesday but appeared flat as she came out for Wednesday’s opening match at the season-ending event for the year’s top eight players.

In her first match in the White Group, the fourth-seeded Azarenka broke four times to take her head-to-head record against Stosur to 5-0.

Sharapova was to face French Open champion Li Na later, while in the Red Group, No. 1-ranked Caroline Wozniacki faced Vera Zvonareva of Russia.