Caroline Wozniacki finished 2014 as the biggest threat to Serena Williams' No. 1 ranking. So far in 2015, Wozniacki hasn't been good enough to get to Serena, although she has lost to Venus Williams twice.

Tennis.com is reporting that the fifth-ranked Wozniacki, who does have a win this year at the Kuala Lumpur Open, has hired four-time Grand Slam champion Arantxa Sanchez-Vicario as part of her coaching team as she enters the clay season.

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Despite her breakout second half to 2014, Wozniacki played in only two clay-court events last year, losing in the second round at the Madrid Open and losing in the first round of the French Open.

"We've been working on more about the mentality going into the game and going onto the surface," Wozniacki said. "You have to hit more balls into the court, you have to run more balls down and change up the pace, and that's really it. My game really should fit with clay, but I haven't had the results I've wanted. I'm trying to do my best to change that."

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To be fair, Wozniacki was dealing with a hamstring injury during clay-court season last year, and her French Open loss came immediately after then-fiancé Rory McIlroy announced their breakup.

But Wozniacki needs some kind of spark as her 2015 has not built upon the success she had when she threw herself into her craft during the second half of last year when she won the Istanbul Open, then advanced to the finals of the U.S. Open and the WTA finals.

Wozniacki's main coach, tennis.com reports, is her husband, Piotr. She has not yet decided whether Sanchez-Vicario will remain with her after the clay season ends; the partnership is in its infancy stages.

But Sanchez-Vicario does have experience in the one major hole in Wozniacki's résumé - a Grand Slam title. The potential is there if their personalities mesh.