Serena Williams is pals with plenty of famous athletes and celebrities, but her closest friend of all, perhaps, has been the one with four legs who achieved a different kind of success: Her beloved dog, Jackie. Sadly, the pampered pooch passed away Tuesday at the age of 16 and Serena posted a heartfelt Instagram tribute to her special friend. Serena wrote that Jackie, whose full name was Jackie Baila Pete Williams, "was still sprinting" up until "a few days ago." The tennis star then added, "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."