Serena Williams to Carolina Wozniacki -- I GOT YOUR BACK!!!

Serena Williams has already chosen sides in the Caroline Wozniacki, Rory McIlroy break up -- telling Caroline she's got her back NO MATTER WHAT.

Wozniacki finally broke her silence Thursday morning ... on Twitter ... saying she's going through a "hard time right now" ever since Rory announced that he dumped her ass after mailing out wedding invitations because he got scared.

In her message on Twitter, the tennis star also noted that she supports the Liverpool soccer team right now because "I know I'll never walk alone."

Soccer Note -- "You'll Never Walk Alone" is the anthem of the Liverpool team, which just came off their best season in 20 years ... while Rory's favorite team Man U just came off one of it's worst.

After she posted the message, Serena Williams tweeted back at her ... writing, "I'll always walk with you #friendsforever."