It all comes down to your definition of "tiers".
If a tier is a group of players whose levels of greatness are hard, almost impossible to distinguish (so no sorting within a tier is possible), then Agassi is a tier below Sampras, because Sampras is clearly, undeniably greater.
This logic also means that Nadal is a tier or two below Federer, by the way.
If by a "tier" you mean a group of players with very comparable achievements (but still sortable) then I would put Agassi and Sampras to the same tier, since they were, indeed, comparable. Nadal, Lendl and Laver would also be in their tier.
If a tier is a group of players whose levels of greatness are hard, almost impossible to distinguish (so no sorting within a tier is possible), then Agassi is a tier below Sampras, because Sampras is clearly, undeniably greater.
This logic also means that Nadal is a tier or two below Federer, by the way.
If by a "tier" you mean a group of players with very comparable achievements (but still sortable) then I would put Agassi and Sampras to the same tier, since they were, indeed, comparable. Nadal, Lendl and Laver would also be in their tier.