Complex sentence: Agassi also held the record for most ATP Masters Series (AMS) shields (since 1990) with 17 (Nadal, Novak Djokovic and Federer have since overtaken him), and has been called the best service returner in the history of the game.
Split into simpler sentences: Agassi also held the record for most ATP Masters Series (AMS) shields (since 1990) with 17. Also, Agassi has been called the best service returner in the history of the game.