One Piece Chapter 5, page 9: CHAPTER 5: THE KING OF THE PIRATES AND THE MASTER SWORDSMAN

Young Zoro complains to his master, Shimotsuki Koushirou, accusing him of favoritism: "SENSEI! ARE YOU TEACHING HER SECRET TRICKS BECAUSE SHE'S YOUR DAUGHTER!? IT'S NOT FAIR!" Koushirou chuckles and replies, "OF COURSE NOT! LOST AGAIN, EH, ZOLO? IT'S ODD BECAUSE YOU ARE ACTUALLY VERY STRONG." Zoro demands, "SENSEI! WHY CAN'T I BEAT THAT GIRL!?" Koushirou gently reminds him, "DON'T FORGET THAT KUINA IS A LITTLE OLDER THAN YOU." Zoro protests, "BUT I CAN BEAT GROWN-UPS!" Later, Zoro trains alone at night, striking a training post repeatedly with his wooden swords. He yells in frustration, "DARN! I CAN'T KEEP LOSING TO THAT GIRL! I'M GOING TO SAIL THE SEAS AND BECOME THE GREATEST SWORD MASTER IN THE WORLD!"