That's what friends are for.

Reuters reported that Serena Williams' BFF, Caroline Wozniacki, handed the world's No. 2 player, Maria Sharapova, a huge setback in her quest to take Williams' No. 1 spot away from her.

Maria Sharapova and Serena Williams are allies against the Russian Tennis Federation president for his hurtful joke, calling Serena and Venus "brothers."

Wozniacki defeated Sharapova 7-6, 5-7, 6-2 on Tuesday in their opening-group match at the WTA Finals in Singapore on Tuesday.

Sharapova did not look like a No. 1 player in the world against Wozniacki, who also defeated her in the fourth round at the U.S. Open.

Maria Sharapova looks to pass Serena Williams for the No. 1 ranking

The Russian committed an unheard of 76 unforced errors in the three-hour, 13-minute contest. Sharapova uncharacteristically buckled under the pressure in the opening set. Sharapova fell behind 3-0 but stormed back for a 5-4 lead with the set on her racket.

Sharapova double-faulted twice in the game but then took a 4-2 lead in the tiebreaker before proceeding to lose the next five points, the last on another double fault.

Wozniacki blew a 5-3 lead in the second set, allowing Sharapova to win the next four games.

Reuters, however, said that Sharapova looked tired in the third set as Wozniacki used here marathon training to run Sharapova all over the court.

"In the third set I feel I stepped it up a little bit, went a bit more for my shots, I am happy to have won," a beaming Wozniacki said.

Sharapova has a chance to surpass Williams for the No. 1 ranking but now has been relegated to the loser's bracket, with the possibility of getting bounced from the tournament before she even gets to Williams.

"In the third set I feel I stepped it up a little bit, went a bit more for my shots, I am happy to have won," a beaming Wozniacki said.

"Just a lot more unforced errors than she did," Sharapova acknowledged.

Williams owns a 16-2 record against Sharapova and has won 15 straight matches against the two.

Do you think Maria Sharapova can still claim the No. 1 ranking? Comment below or tell us @SportsWN.