Psa 143:1 O LORD, hear my prayer, listen to my cry for mercy; in your faithfulness and righteousness come to my relief. The question is can we separate the law of God with the covenant promises? The law in the OT was always a covenant -law. Why was the law and the covenant connected? Because God was putting Himself on notice that He alone would fulfill the requirements of the law in order to show Himself faithful in His covenant promises. Christ was covenant keeper. 
So the Psalmist uses the covenant promises in defense of Israel. In this case he appeals to God in the petition according to God's faithfulness and righteousness. Now this covenant is eternal. So the Psalmist always petitions God for the proof of His covenant promise to be seen in Israel on the perfect we or of the Mediator on behalf of Israel. The Covenant law giver created the world and promises to govern His creation by protecting His covenant people. The Psalmist petitions the kingdom on earth in opposing the nation's by pronouncing the law that curses the wicked, the curses that remove all opposition ,the decrees that establish God's kingdom on earth the statutes that establish Israel as a nation of laws ,and the promises that are gifts of God in ruling the world.