He's not guaranteed to play the night session. Whilst in 2011 and 2010 the top two seeds started in the evening, in 2009 the number 2 seed (Rafa) played in the afternoon.
Whilst the night match has better television viewing, they don't necessarily put the biggest drawing match on then. In 2010, Federer played Murray during the afternoon. As you only know which match is scheduled a couple of days before, the cynic in my thinks this was because they had unsold tickets for that session.
My String Journey: http://tt.tennis-warehouse.com/showthread.php?t=362403