In the final set, she was trailing her opponent 4-5 and had to break serve to stay in the match.

trailing 是动词 trail 的现在分词形式。trail 本意是"跟随、追踪"，在体育比赛中引申为"比分落后"的意思。 这里 trailing 表示"处于落后状态"，描述比赛中的比分情况。