(CNN) -- World No.1 Caroline Wozniacki survived the loss of the opening set to beat Poland's Agnieszka Radwanska 5-7 6-2 6-4 in their Red Group opener at the WTA Championships in Istanbul Tuesday.
Wozniacki looked in trouble after losing the opening set, being broken twice by Radwanska, who is a newcomer to the prestigious end of season tournament.
But Wozniakci, with the continuous prompting of her Polish-born father and coach Piotr, turned it around after falling another break down in the second set.
She immediately broke back as Radwanska, the last qualifier in the elite eight-strong field, lost four games in a row then called the trainer.
She received treatment to her shoulder but continued to struggle in the deciding set to fall 4-2 behind.
But as Wozniacki began to make errors, she broke back to level at 4-4 and set up a tense finale.
Wozniacki steadied to lead 5-4 then forged a match point on Radwanska's service, which was brilliant saved.
A second soon came and Radwanska struck her return into the net to give the Dane a hard-fought victory after two hours 39 minutes.
She needs to win one more group match and qualify for the final of the $4.9 million tournament to be crowned World No.1 for the second straight year.
But a grand slam win still eludes her.
"I believe if I keep doing the right things, the outcome that I want will come," Wozniacki said after her win. "It's a question of keeping your focus right and the results will look after themselves."
In the first match in the Red Group, Wimbledon champion Petra Kvitova showed herself in fine form on her debut in the WTA Championships with a 6-2 6-4 win over Russian Vera Zvonareva.
But there was defeat for Maria Sharapova as she began her White Group campaign against U.S. Open champion Samantha Stosur of Australia in the final match of the day.
Sharapova was returning to action after injury and was beaten for the first time in 10 meetings by Stosur, going down 6-1 7-5.
Second seed Sharapova battled hard after losing the opener, breaking to lead 3-0 in the second.
But Stosur broke back and then made another breakthrough to lead 6-5.
Russia's Sharapova came close to forcing a tiebreaker but facing match point, overhit her return and went down to a one hour 33 minute defeat.