Petra Kvitova of the Czech Republic hits a return to Vera Zvonareva of Russia during their WTA tennis championships match in Istanbul, October 25, 2011. [Photo/Agencies]

ISTANBUL - Caroline Wozniacki gave another gritty demonstration of why she has enjoyed a virtual year-long stranglehold on the world number one ranking with a 5-7 6-2 6-4 defeat of Agnieszka Radwanska in her opening match at the WTA Championships on Tuesday.

With world number two Maria Sharapova losing to Samantha Stosur for the first time in her career in the day's late match, the pugnacious 21-year-old Wozniacki is inching closer to a second consecutive year end at the top of the tree despite still waiting for a grand slam title.

Her hard-fought victory meant Denmark's most-photographed athlete, not least because of her romance with Northern Irish golfer Rory McIlroy, joined Petra Kvitova, who has an outside chance of de-throning Wozniacki, at the top of Red Group.

Czech leftie Kvitova, whose stunning Wimbledon triumph helped power her up to third on the latest WTA ranking list, showed a few early and late nerves on her tournament debut as she opened proceedings inside Istanbul's cavernous Sinan Erdem Dome with a 6-2 6-4 defeat of Russia's Vera Zvonareva.

Caroline Wozniacki of Denmark celebrates after defating Agnieszka Radwanska of Poland during their WTA tennis championships match in Istanbul, October 25, 2011.??[Photo/Agencies]

"I was nervous because it was my first time playing at the Championships," Kvitova, the first Czech to qualify since Jana Novotna in 1998, told reporters after allowing a 4-1 lead to slip in the second set but then recovering her poise.

While her match was played out to polite applause from the hardly-engrossed 10,000 crowd, the middle match of the day set the standard for $4.9 million year-ending showpiece which will be played in the majestic Turkish capital for three years.

Wozniacki, whose unerring consistency has earned her a total of 54 weeks as No.1, was made to play catch-up against Radwanska who gained the initiative early with some beautifully-timed effortless winners.

Catch up she did, however, moving 0-40 ahead when Radwanska served at 4-5 in the first set only to blow all three set points. World number eight Radwanska seized on her let-off, breaking in the following game and then clubbing away a forehand to pocket a 64-minute first set.

With her father Piotr offering some animated words of wisdom at virtually every change of ends, Wozniacki reeled off four games from 2-2 in the second set to level the match and appeared to be in control in the deciding set when a stunning backhand winner to end a superb rally put her 4-2 ahead.

However, she gifted a break back with a double-fault in the next game and Radwanska, who had her right shoulder taped, began to play some inspired tennis to drag herself back to 4-4.

Wozniacki squandered one match point but a tired-looking Radwanska forehand into the net at 4-5 ended the duel. Should Wozniacki win another round-robin match and reach the final she will be guaranteed the top ranking going into 2011.

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.