I'm pretty sure you can abort unless it is a tournament.
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Once you have accepted a challenge the game starts immediatley. You have until your time runs out to move. If you don't make a move before that time, you lose. If this is live chess, it should be automatic. If it is correspondence, then it can happen automatically if your opponent has it configured to auto-claim on time, or when manually claim it if not.

Its the team match you joined. You play one game as white, one as black. Your current is the team rematch
http://www.chess.com/groups/team_match.html?id=45827
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