Final answer:
The sentence in question uses a secondary source because ESPN is reporting on the event of Andre Agassi's retirement without having directly participated in it.
Step-by-step explanation:
The sentence 'Tennis great Andre Agassi will retire at the end of the year, ESPN reported on its Web site Thursday' uses a secondary source. A secondary source provides information that was created later by someone who did not experience first-hand or participate in the events or conditions you're researching. In this case, ESPN is reporting on the news of Andre Agassi's retirement, but it did not directly participate in the event (Agassi's decision to retire); rather, it is relaying information that has been announced elsewhere.