Serena Williams: My Dog Is Dying ... And It's Breaking My Heart

10:58 AM PT -- More sad news ... Serena just announced her beloved Jackie has passed away.

Williams wrote a touching note about her dog ... saying, "My special friend in which I got at 17 (2 weeks before I won my very first Grand Slam) left me today."

"She was 16 years young and up until a few days ago was still sprinting. Her poor body gave out this morning and she had a way of telling me it was time for me to be brave and let her go."

"With my dad by my side we were able to say a loving goodbye. She was with me from 1999 until today and I miss her so much."

"I got out of the shower this afternoon and she was not there to lick my leg as she always did everyday to remind me how much she loved me. I feel so lucky to have such a special friend. Give your dog, cat, pet a big hug. #breakingheart #bff Jackie Baila Pete Williams I will miss you and your memory will live forever."

Serena Williams is afraid she's about to lose the "love of her life" ... her beloved dog Jackie -- her best friend for 16 years.

Williams opened up about Jackie in an emotional video ... fighting back tears as she explained that Jackie is "struggling" with her health and it's breaking her heart.

"I just want to share this with everyone 'cause she's getting old and she's really struggling and she means the world to me ... and she's made my life better."

If you've ever had a pet you cared about ... you can probably relate to how Serena's feeling. Sucks.