Addressing God as the Father seems pretty normal - not the case in Jesus' day
Sometimes David prayed:
...but nowhere will you find David addressing the living god as "Father"
Gabriel was sent to respond personally to Daniel's great prayer in Daniel 9 - yet he did not address God as Father
Patriarchs, the prophets, and the priests of Israel were all men of prayer - never addressed their prayers to God as "Father"
Jesus gave believers the right to become children of God
We begin our time in prayer with a father-child relationship