Alfred Hitchcock - The Ring (1927)

Jack Sander is a carnival sideshow attraction, he boxing any taker, their goal to last one round in the ring with him, hence his moniker "One Round" Jack. It is the job of his pretty assistant, his unofficial fiancee Mabel, not only to sell tickets but to provide encouragement to spectators to take on Jack. When Jack faces an opponent who not only lasts more than one round but knocks him out in the fourth round, he believes his career at the carnival is over. However, he eventually learns his opponent was champion boxer Bob Corby, who came to test out Jack's boxing skill first hand in looking for a full time sparring partner. With one hurdle left to overcome for Jack to get the position, Jack, in this association with Bob, would also allow him the opportunity for a full-fledged boxing career. When Jack makes the statement that he would fight for Mabel, he was not aware at the time that he may literally do so in the ring against Bob, an attraction between Bob and Mabel which has existed as early as they setting eyes upon each other outside the carnival tent.