Touch Battle

"It was a tough battle out there," she told reporters. "We had a lot of long rallies and I'm very happy to pull through." The atmosphere is very nice here, it's a huge crowd.

Sharapova, who until last week was still unsure whether she would be fit to play after twisting an ankle before the China Open, was caught cold by Stosur in their White Group match which finished at nearly midnight local time.

Maria Sharapova of Russia hits a return to Samantha Stosur of Australia during their WTA tennis championships match in Istanbul, October 25, 2011. [Photo/Agencies]

However, she took some positives from a 6-1 7-5 defeat in her first match at the year-ending showpiece since 2007, since when a shoulder injury threatened her career.

"The good thing is that I gave myself a chance and I did everything I could just to be here," the three-times grand slam champion, who plays China's Li Na on Wednesday, told reporters. "I'm happy that I still have a couple of matches to go.

"She was more explosive than me today," added Sharapova who had won all her previous nine matches with Stosur.

Stosur, playing at the Championships for the first time in singles, saved three break points when serving for the match at 6-5 before finally clinching it on her second match point when Sharapova blazed a return long.

Samantha Stosur of Australia hits a return to Maria Sharapova of Russia during their WTA tennis championships match in Istanbul October 25, 2011. [Photo/Agencies]

The Australian can take a huge step towards the semi-finals if she beats Victoria Azarenka in Wednesday's first match.