If it is a matter of strength, He is indeed mighty! If it is a matter of justice, who can summon Him?
What Job had said of man's utter inability to contend with God he here applies to himself, and in effect despairs of gaining his favour, which (some think) arises from the hard thoughts he had of God, as one who, having set himself against him, right or wrong, would be too hard for him.
Commenting on Job 9:14-21
If I justify myself,.... Seek for justification by his own righteousness, trust in himself that he was righteous, say that he was so, and pronounce himself a righteous man, what would it signify? mine own mouth shall condemn me; the words of it being sinful, vain, idle, and frothy; and if a man is to be justified, and condemned by his words, he may be...
UMBREIT takes these as the words of God, translating, "What availeth the might of the strong?" "Here (saith he) behold! what availeth justice? Who will appoint me a time to plead?" (So Jer 49:19). The last words certainly apply better to God than to Job. The sense is substantially the same if we make "me" apply to Job. The "lo!" expresses God's swift readiness for battle when challenged.