Maria Sharapova had her chance. The role of Serena Williams' rival now is being played by Caroline Wozniacki.

With apologies to Simona Halep, the No. 4 player in the world who handed Serena her most lopsided loss in 16 years and who faces Williams in the WTA Finals on Sunday, Wozniacki has emerged as her best friend's biggest threat to the No. 1 ranking.

Caroline Wozniacki defends best friend Serena Williams' No. 1 ranking by knocking off Maria Sharapova.

"This really sucks." Wozniacki said, according to tennis.com. "You know, being so close and still losing, it really sucks."

Serena improved to 10-1 all-time vs. Wozniacki, but the Dane who has resurrected her tennis career in the summer of 2014, got much closer to Williams than she did when the two met in the U.S. Open final in September.

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Williams, the 18-time Grand Slam champion, won that match 6-3, 6-3 in just 75 minutes. She had a much tougher time on Saturday against the Dane.

"She was playing me different than she had in the last four or five times we played this year, so I just had to adjust to that," Williams said. "I thought she was playing totally different. She had a whole new strategy ... it clicked today. I thought, 'Ha! I got you now.' "

Sharapova came into the WTA Finals ranked No. 2 and with a shot of overtaking Serena for the No. 1 ranking at season's end, despite her 15-match losing streak to Williams. The Russian, however, couldn't advance out of the round-robin portion of the event.

Wozniacki, the No. 1 player in the world in parts of 2010 and 2011, had dropped to No. 14 earlier in the season appeared ready to give up the game to start a family with golfer Rory McIlroy, to whom she was engaged. But McIlroy broke off the engagement just before the French Open.

The breakup fueled Wozniacki to rededicate herself to her tennis, and the results have been positive. She won the Istanbul Cup in July for her first win of the season, and then she made her run at the U.S,. Open.

Ironically, Williams was the one that rushed to Wozniacki's aid after the breakup. Now, Wozniacki could loom at the player that keeps Williams from breaking Steffi Graf's all-time record of 22 Grand Slams.

Who do you think is Serena Williams' biggest rival? Comment below or tell us @SportsWN.