SINGAPORE -- Caroline Wozniacki credited her training for next month's New York Marathon as a key factor in her victory over Maria Sharapova in a grueling three-set match in the WTA Finals on Tuesday.Wozniacki won 7-6 (4), 6-7 (5), 6-2 to beat Sharapova for the second-straight time, having also won in the fourth round of the U.S. Open. Tuesday's result in the round-robin match dealt a major blow to Sharapova's hopes of claiming the year-end No. 1 ranking.Wozniacki, who has always been regarded as one of the fittest players on tour, said her additional running work in preparation for the marathon had taken her stamina to a new level and also given her added belief in Tuesday's 3 hour, 15-minute match."I just feel like right now with all the training and running I've been doing I can keep being out there and keep running," Wozniacki said. "I kept thinking to myself out there in the third set, If you're going to get tired now, how are you going to get through this marathon? You better keep going."As much as she believed the training had helped her aerobic fitness, she said the marathon will be a one-time-only affair and not one that she would recommend to other players."To do it year in and year out, I don't think it's possible," Wozniacki said. "Our bodies get a beating already. I don't think it's necessary to do this again while I'm on tour."No such stamina was required in Tuesday's other match as Agnieszka Radwanska had a comfortable 6-2, 6-3 win over Wimbledon champion Petra Kvitova.It was an impressive performance by the Pole, who had been beaten in straight sets the past two times she had played Kvitova. It was the fourth successive year in which the pair had faced each other in their opening matches 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 had 12 breaks of serve, with 35 unforced errors.The defeat in her opening round-robin match means Sharapova now needs to at least reach the final in Singapore and hope Serena Williams loses her two remaining group matches if she is to overtake the American and finish the season with the top ranking for the first time.Having staved off a set point against her in the 10th game, Wozniacki took the first set by winning the final five points of the tiebreaker.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 during the middle of a point and remained on for the rest of the game.The Dane recovered quickly after dropping the set, breaking Sharapova early in the third and then twice more to end it.Sharapova was frustrated by her error-strewn performance, blaming her mistakes rather than Wozniacki's play for her two consecutive losses to the Dane."It's happened the last couple of times I played against her," Sharapova said. "The chances that you have when you're serving for a set, it's in your hands to finish that off."When you give those chances back, a player that's top 10 in the world and has the experience as Caroline does, you're not just going to run away with the match."The losses by Sharapova and Kvitova have thrown the White Group wide open, with Radwanska's convincing victory highlighting her title credentials.The Pole has not performed at her best in past appearances at the season-ending event, having been eliminated at the group stage in three out of four years."I feel much better than last year at the championships," Radwanska said. "I haven't played many matches in the past two weeks, which has helped."I've learnt my lessons from past years about how to play at my best."The match started after 11 p.m. local time, when Kvitova said "normally I am sleeping", and began as an even contest. But Radwanska managed breaks in the Czech's last two service games of the opening set, and then won the final three games of the second set to record a convincing victory.Kvitova, who won only two points on her second serve during the second set, said Radwanska did not produce "anything really special" but she struggled to read the pace of the court."Sometimes was so fast and so slow, so was really difficult for me to find the timing," Kvitova said. "My serve was OK, but she returned a lot of balls back, which is always difficult for me, to then play rally with her."In Wednesday's matches, Williams will face Simona Halep while the other Red Group match will be Ana Ivanovic against Eugenie Bouchard.