She defeated all her opponents in last year's tennis tournament and became the champion.

champion 是名词，来自法语，原意是"战场上的胜利者"，现在指"比赛中的第一名"或"冠军"。这个词也可以用作动词，表示"捍卫、支持"的意思，但在本句中是用作名词。