I was also upright before him, and I kept myself from mine iniquity.
I was also upright before him - literally, with Him (Genesis 17:1; Deuteronomy 18:13). See God's own testimony to David (1 Kings 14:8) "My servant David kept my commandments, and followed me with all his heart, to do that only which was right in mine eyes;" also 15:5.
And I kept myself from mine iniquity - i:e., from the iniquity to which I am liable. Herein he shows himself as not perfectly righteous, but a believer needing watchfulness and prayer to keep down his natural corruption (cf. note, Psalms 17:4). See an instance of his going to the verge of sin against his enemy Saul, but shrinking back from it in time (1 Samuel 24:4-7; also 26:23-24, which compare with Psalms 18:20 here). 'What he there hopes for on the ground of his righteousness, that he here marks as granted to him on the same ground' (Hengstenberg).