ISTANBUL (AP) Top-ranked Caroline Wozniacki rallied to defeat Agnieszka Radwanska of Poland 5-7, 6-2, 6-4 Tuesday in their opening round-robin match at the WTA Championships.

Wimbledon champion Petra Kvitova and U.S. Open winner Sam Stosur also began the group stage with a victory at the season-ending tournament. Kvitova beat Vera Zvonareva of Russia 6-2, 6-4 in the Red Group - which also includes Wozniacki and Radwanska - while Stosur defeated Maria Sharapova 6-1, 7-5 in the White Group.

Wozniacki wasted a 4-2 lead in the third set, but broke again when Radwanska netted a forehand on the Dane's second match point.

Wozniacki had also wasted three set points in the first set of a match where both players struggled to hold serve on the indoor hard court. The second set alone featured six breaks of serve, with Wozniacki getting four of them.

``It was a bit frustrating not to get the first set after having three set points, but I managed to keep my head cool and just fight back,'' Wozniacki said.

Making her first appearance at the season-ending tournament for the top eight players of the season, Kvitova wasted a 4-1 lead to be even at 4-4 in the second set but broke again immediately before wrapping up the win.

Stosur breezed through the first set without facing a break point against the second-ranked Sharapova, who played her first match since pulling out of the quarterfinals in Tokyo with an ankle injury last month.

But Sharapova found her rhythm early in the second set, where she took a 3-0 lead and had a chance to break again in the fourth game. But Stosur held and evened the score before breaking again for 6-5.

It did take me a while to get going,'' Sharapova said.It's obviously tough to come back after a month and compete at this level. But the good thing is that I gave myself a chance, and I did everything I could to even be here.''

Sharapova finished with six double faults and only hit 59 percent of her first serves - compared to 74 percent for Stosur.

The top two players in each group advance to the semifinals.

This is the first time the tournament is held in Istanbul, after being staged in Doha, Qatar, the past three years.

The surface is a little bit weird,'' Radwanska said.It's pretty slow. So you really have to be used to it. I just hope I can play better in the next match.''