Li Na of China celebrates a point during her WTA tennis championships match against Maria Sharapova of Russia in Istanbul, Oct 26, 2011. [Photo/Agencies]

ISTANBUL - Maria Sharapova's hopes of ending the year as world number one virtually evaporated on Wednesday when she was beaten 7-6 6-4 by China's Li Na at the WTA Championships.

The Russian, who arrived in Istanbul still with a shot at toppling Caroline Wozniacki, fell apart in the first set tiebreaker on her way to a second consecutive White Group defeat at the $4.9 million year-ending tournament.

Samantha Stosur, who beat Sharapova on Tuesday for the first time in 10 attempts, opened proceedings on Day Two in the Sinan Erdem Arena but came down to earth with a bump as she lost 6-2 6-2 to fired-up Belarussian Victoria Azarenka.

Denmark's Wozniacki can seal the year-end No 1 ranking for a second consecutive year if she beats Russia's Vera Zvonareva in the final match of the day.

World number two Sharapova, playing at the Championships for the first time since 2007, almost missed the event with an ankle injury and her lack of recent match practice was evident again as she let tournament debutant Li off the hook.

Maria Sharapova of Russia reacts during her WTA tennis championships match against Li Na of China in Istanbul, Oct 26, 2011. [Photo/Agencies]

Sharapova led 4-1 in the opening set and after seeing her lead evaporate she then moved 4-0 ahead in the tiebreak only to lose the next seven points with a run of unforced errors.

Li, who beat Sharapova in the French Open semi-finals this year on her way to claiming a first grand slam singles title for an Asian-born player, moved into a 5-2 lead in the second set before an attack of the jitters almost proved costly.

Serving for the match at 5-4 Li slipped 15-40 behind as Sharapova blazed away but she held her nerve to claim victory at the first attempt when her opponent hammered a backhand return over the baseline.

While it is still mathematically possible for Sharapova to finish in the top two in the group her chances are hanging by a thread with a match against ther in-form Azarenka to come.