Q. What are the requirements and rules for player participation for an ATP World Tour 500 tournament?
A. Top 30 players (based on 2011 year-end South African Airways ATP Rankings) must play a minimum of four 500 level tournaments during the calendar year, including at least one event following the US Open (Monte Carlo Masters 1000 event will count towards the minimum of four and all penalties apply):
-- A 0-pointer for each event less than four played.
-- A 0-pointer for withdrawing from any 500 tournament after the acceptance list is out.
-- A 0-pointer for not playing at least one event after the US Open.
-- A 0-pointer can be appealed by a player to the same tribunal formed to hear 1000 suspension appeals.
-- No suspensions or fines (including no withdrawal and late withdrawal fines).
This is from the ATP rankings FAQ
, ATP requires a minimum of 4 500's tournaments played, monte-carlo counts as one of them, it means djokovic will have 3 500's played(dubai, monte-carlo, beijing), so he will receive a 0-pointer for not playing basel.
Federer played rotterdam and dubai, he already received a 0-pointer for tokyo, so if he plays basel he wont receive another 0-pointer.