Serena Williams' toughest foe in 2015 will be Serena Williams.

That was the message delivered by the reigning 2014 Australian Open women's champion just prior to the start of the first Grand Slam event in Melbourne in 2015.

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Retired tennis player Li Na, who captured last year's first Grand Slam event, told the New York Times that despite all the rumblings from established and up-and-coming challengers, she anticipates that the ball remains in Williams' court.

"Serena is always tough," Li told the Times. "As long as she wants to be at the top, she will be, because she is such a good player with a mind of a champion."

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Which has been the curse for Williams, the 18-time Grand Slam champion. Most tennis analysts believe that when Williams is on her game, no one can beat her, citing her 2013 season as evidence. Serena went 78-4 that year, winning 11 titles.

She won the French and U.S. Opens that year. Ironically, Williams capped her 2013 by defeating Li 2-6, 6-3, 6-0 in the season-ending WTA Championships.

Williams, however, failed to match her 2013 feat the following season. She still had an impressive 52-8 record in 2014, but she failed to get past the fourth round in any of the first three majors of the year. Serena managed to win "only" seven tournaments in 2014, including four after her infamous Wimbledon doubles match with partner Venus Williams.

A video showing Serena's inability to serve or even bounce the ball on the surface with the racket went viral. She finally consented to forfeiting the match after just three games.

Williams also has gotten off to a somewhat suspicious start in 2015. She lost two of three singles matches at the Hopman Cup. She lost two of her four singles matches at the Hopman Cup, including a a 6-1, 6-1 thrashing at the hands of No. 7 Eugenie Bouchard in 50 minutes. She also lost to Agnieszka Radwanska for the first time in her career.

If Li is accurate about Serena's ability to dominate, next week's Australian Open is as good a starting point as any for Williams to prove her retired rival right.