One Piece Chapter 51, page 9: Roronoa Zoro Falls Into the Sea

Zoro's flashback continues with his promise to Kuina: "Someday, one of us will become the world's greatest swordsman! Let's compete to see which of us will be that person! It's a promise! I'll become even stronger for her sake! I swear I'll become the world's strongest swordsman whose name is famous enough to reach even the heavens!" He also remembers meeting Johnny and Yosaku, who introduced themselves: "We're the bounty hunters, Yosaku and Johnny! We're impressed by your skill! Let us work together with you!" Back in the present, Mihawk looks at Zoro's fierce determination and asks, "What burdens you so? What do you still desire at the extent of your strength, weakling...?"