Maria Sharapova's chances of claiming the season-ending No. 1 ranking were hurt Tuesday after a 7-6 (4), 6-7 (5), 6-2 loss to Caroline Wozniacki at the WTA Finals.
Sharapova had 15 double-faults, including two while serving for the first set, and 76 unforced errors. Wozniacki finished the match, which has 12 breaks of serve, with 35 unforced errors.
It was Wozniacki's second straight victory over Sharapova. The Dane also won in the fourth round of this year's US Open.
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 top ranking for the first time.
In Tuesday's other White Group match, Wimbledon champion Petra Kvitova will face Agnieszka Radwanska.
After forcing the tiebreaker in the first set, Wozniacki trailed 4-2 but won five straight points to take the set.
Wozniacki went up a break early in the second, but relinquished the advantage in the sixth game as she complained to the chair umpire about swirling lights which suddenly came on mid-point and remained on for the rest of the game.
The Dane recovered quickly after dropping the set, however, breaking Sharapova early in the third and then again to end it.
All four of Sharapova's titles in 2014 required three sets, but it was Wozniacki, who is planning to run the New York Marathon next month, that had held on for victory this time.