Caroline Wozniacki shocked second seed Maria Sharapova 7-6 (7-4) 6-7 (5-7) 6-2 in a thrilling opener to the White Group at the WTA Tour Finals in Singapore.

In a match which featured 12 breaks of serve, Sharapova served 15 double-faults, including two while serving for the first set, and made a total of 76 unforced errors.

The eighth-seeded Wozniacki was hardly a model of consistency herself, with 35 unforced errors.

After forcing the tiebreaker in the first set, Wozniacki trailed 4-2 but won five straight points to take the set.

The Dane went up a break early in the second, but surrendered the advantage in the sixth game after complaining to the umpire about swirling lights which suddenly came on mid-point and remained on for the rest of the game.

Wozniacki recovered quickly after dropping the set, breaking Sharapova early in the third and then again to wrap up the match.

Happy

"In the third set I feel I stepped it up a little bit, went a bit more for my shots, I am happy to have won," Wozniacki said.

It was Wozniacki's second straight victory over the five-time Grand Slam champion after she also won in the fourth round of this year's US Open during her run to the final.

Sharapova needs to at least reach the final in Singapore and hope Serena Williams loses her two remaining group matches to overtake the American and finish the season with the No 1 ranking for the first time.

There was also an upset in Tuesday's other White Group match, as Polish sixth seed Agnieszka Radwanska beat third-seeded Wimbledon champion Petra Kvitova of the Czech Republic 6-2 6-3.

It was an impressive performance from Radwanska, who had been beaten in straight sets on the last two occasions the pair met.