The excerpt is titled "I Hate Tennis" and covers a lot of ground from the book...
Sections begin with Agassi's early years first with the relentless pushing courtesy of his overbearing father and then his money match against football great Jim Brown at age 9. Later portions of the book here get into Agassi's Wimbledon championship at 22 and now much publicized (as a result of this book) Crystal Meth usage.
Finally, the excerpt wraps up with Agassi's final US Open... with both family and thoughts of family around him.
To this point of family, the section on Agassi's 60 something year old father meeting Steffi Graf's similarly aged father is so amazing you almost question if it happened as described. The gist was these two tyrant "sports father" types getting so competitive and worked up that they practically came to blows... with the argument between the two not having the benefit of a shared language.
Wild stuff and while the book itself is I imagine an interesting read throughout, this SI excerpt provides a good condensed view.