A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 143:1In structure and style, like the preceding (Psalms 104-142), this Psalm is clearly evinced to be David's. It is a prayer for pardon, and for relief from enemies; afflictions, as usual, producing confession and penitence. (Psa 143:1-12)
in thy faithfulness . . . and . . . righteousness--or, God's regard to the claims which He has permitted His people to make in His covenant.
psa 143:2enter . . . judgment--deal not in strict justice.
shall no . . . justified--or, "is no man justified," or "innocent" (Job 14:3; Rom 3:20).
psa 143:3The exciting reason for his prayer--his afflictions--led to confession as just made: he now makes the complaint.
as those that have been long dead--deprived of life's comforts (compare Psa 40:15; Psa 88:3-6).
psa 143:5The distress is aggravated by the contrast of former comfort (Psa 22:3-5), for whose return he longs.
a thirsty land--which needs rain, as did his spirit God's gracious visits (Psa 28:1; Psa 89:17).
psa 143:7spirit faileth--is exhausted.
psa 143:8(Compare Psa 25:1-4; Psa 59:16).
the way . . . walk--that is, the way of safety and righteousness (Psa 142:3-6).
psa 143:9(Compare Psa 31:15-20).
psa 143:10(Compare Psa 5:8; Psa 27:11).
land of uprightness--literally, "an even land" (Psa 26:12).
psa 143:11(Compare Psa 23:3; Psa 119:156).
psa 143:12God's mercy to His people is often wrath to His and their enemies (compare Psa 31:17).
thy servant--as chosen to be such, entitled to divine regard.