The assassin and the lone guard is exactly the kind of edge case where following the rules strictly can lead to strange outcomes, so it's time for a DM ruling. If the assassin managed to sneak up on the guard I'd simply let him make an attack roll with all benefits of surprise and attacking from hiding. If the guard survives he shouts out an alarm, initiative is rolled and regular combat rounds start.
This would not create any presedence that being the first to declare an attack would get you a "free attack" in other combat situations or anything like that, it would be a one time ruling for this specific circumstance.
This would not create any presedence that being the first to declare an attack would get you a "free attack" in other combat situations or anything like that, it would be a one time ruling for this specific circumstance.