The first fight they get into as a crew after Morgan; Buggy beats Zoro and Zoro has no chance to win, while Luffy literally eats Buggy's attack even when his hands were tied, then wounded Zoro realizes he has no chance against Buggy, he escapes by carrying Luffy’s cage.
Then Luffy beats Buggy, while Zoro beats Buggy's chief of staff Cabaji, who was equal to Buggy's first mate Mohji (which Luffy one-shotted).
That was the beginning of the manga, did you skip that part?
Also Oda originally planned to make Zoro one of the sidekicks of Buggy