Philadelphia will be among the 12 stops on a new nationwide Champions Series tennis circuit to debut this fall, organizers announced yesterday. Information on venues, dates, fields, tickets and television will be announced later, organizers said. Andre Agassi, Pete Sampras, John McEnroe and Jim Courier will headline the circuit, with each of the one-night shootout events guranteed to draw three of the legends and one other former champion. The four players will compete in one-set semifinals with the winners in an eight-game pro-set championship match. Players will compete for ranking points and a $1 million bonus be shared by the top three finishers at the end of the season. Events will take place from Sept. 22 to Oct. 22. Tour stops in Los Angeles and Chicago also were announced yesterday.