The incumbent regime lead by Slobodan Milosevic reported the victory too, but the report lacked the expected exuberance, and it morosely admitted to the need for the second round - a sign that they lost and now unwilling to admit to that, they are also insecure to lie big (like telling that they won with overwhelming majority).
The other sign that the regime gave up on trying to convince people in their victory is that they stopped updating its web site with election results fixed at the 3.5% of the vote count.
Therefore, we may conclude that Vojislav Kostunica is now the President of FR Yugoslavia, and should be regarded as such in all future correspondence.