Losing that match was the worst he had experienced in his career.

the 是定冠词，用于特指某个特定的事物或情况。 worst 是形容词 bad 的最高级形式，意思是"最坏的、最糟糕的"。 the worst 表示"最糟糕的那个"，在这里特指"职业生涯中最糟糕的经历"。