Why did Rustum did not reveal his identity at the start of the mortal combat?

From the story, Sohrab and Rustum? If so, he does not reveal his identity because he isn't aware that Sohrab is his son. Sohrab challenges the Persian army to choose their champion, so that his fame (as victor) will come to his father's attention. Sohrab and Rustum have never met, thus when they meet on the field of battle, they stand as representatives for their warriors.