Job cries to the avenger of blood to avenge his innocence. He is a martyr, and feels that his blood must cry for vengeance (Genesis 4:10 *, Revelation 6:10). Job arrives at the astounding thought that God will be his avenger, though it is God that slays him. We have noticed how in Job's bitter complaint against God, the thought that the God, who had loved him in the past, will one day turn to him once more, had again and again broken through (Job 7:8; Job 7:21; Job 14:13-15). Job now sets the God of the past and the future against the God of the present, one side of God against another, God against Himself (Job 16:21). God is his witness (Job 16:19). Davidson translates advocate and says, There was no difference between advocate and witness in the Hebrew courts, the part of a witness being to testify on behalf of one and see justice done him.
Related Commentaries of Job 16:18
Job 16:18-21
18 O earth, cover not thou my blood, and let my cry have no place.
19 Also now, behold, my witness is in heaven, and my record is on high.c
20 My friends scorn me: but mine eyed poureth out tears unto God.
21 O that one might plead for a man with God, as a man pleadeth for his neighbour!