They friendship revived both their careers and sparked a rivalry that could keep both atop the world ranking for months to come.

But will that budding rivalry tear apart their friendship?

Caroline Wozniacki now Serena Williams' biggest rival?

Tennis.com reported that Serena Williams and Caroline Wozniacki went to a Mariah Carey concert in Singapore just hours before their semifinals match at the WTA Finals, which Williams won in a third-set tiebreak.

Williams smashed her racket during the opening set of their match.

But Wozniacki clearly was frustrated after losing the tiebreaker 8-6 in the third to her best friend.

Serena Williams' BFF Caroline Wozniacki takes out Serena's rival, Maria Sharapova, at WTA Finals pool play.

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

Williams went on to rout Simona Halep 6-3, 6-0 in the WTA Finals on Sunday, just four days after Halep defeated Williams 6-0, 6-0 in their pool-play match.

Serena played the role of heroine after Wozniacki was stunned by the sudden breakup with fiancé Rory McIlroy. Williams whisked Wozniacki away for a weekend in Miami, that may have been as therapeutic for Williams as it was for Wozniacki.

Wozniacki's results were immediate. She began training to run for a marathon, which helped her tennis fitness immensely. She won a tournament in Istanbul, which propelled her to the U.S. Open final, where she beat Maria Sharapova on the way to a straight-sets loss in 75 minutes to Williams in the final.

Serena, who's rumored relationship with coach Patrick Mouratoglou was also rumored to have ended, took a bit longer to turn around her season. But her weekend with Wozniacki was a chance for her to get away from the grind and what had been a frustrating season with early losses in the Australian Open and the French Open.

After her bizarre exit from Wimbledon, Serena finally found her game and won her only Grand Slam of the Year at the U.S. Open (her 18th overall). No doubt, watching Wozniacki resurrect her career had to have had an impact on Serena's own game.

But now that the two could be playing for championship in the months, or even years, to come, their friendship could be tested. Especially if Wozniacki starts to win some of these matches. How will Serena react if Wozniacki breaks through and starts to win some of these matches?

Currently, Williams owns a 10-1 record against Wozniacki.

Whether the friendship becomes strained, Wozniacki's emergence could be what Serena needs to push her past Steffi Graf for the most Grand Slam wins of all-time.

Do you think Serena Williams and Caroline Wozniacki remain close friends if they continue to play each other in semfinals or finals of big-time tournaments? Comment below or tell us @SportsWN.