Rocky made both Stallone and director John G. Avildsen into champions. Just like Rocky was down on his luck before having a shot at the title, Avildsen, who had small critical success with Joe (1970) starring Peter Boyle and Susan Sarandon, had been fired from Serpico and Saturday Night Fever.
But along came Rocky, and an Academy Award for Best Director for Avildsen. Avildsen later returned for Rocky V (1990), but he preferred the low-budget original, saying it forced the team to make more creative decisions, like filming Rocky and Adrian's date inside an empty ice-skating rink instead of a busy restaurant. (Source)
Since Rocky, Avildsen has a thing for underdogs kicking butt: he also directed three Karate Kid movies. Although Avildsen was big in the '70s and '80s, after two Worst Director Razzie nominations, for Rocky V and The Karate Kid: Part III (1989), Avildsen only directed three more films.