This verse is the second phrase of the couplet:
Yea, cry unto him for mercy for he is mighty to save.
Yea, humble yourselves, and continue in prayer unto him.
The first two clauses are parallels in the position of the supplicant before the king. A person will approach the king asking for some benefit, a benefit that the supplicant does not deserve, but which is in the power of the king to grant. Thus the two first phrases emphasize the necessary attitude of the supplicant. The person must approach the king in humility. One who cries for mercy is necessarily in a humble position, for the request is one that requires the supplicant to be understood as subservient to the king.
The second clause of each of the couplets refer to the position of the king. The first emphasizes the ability of the king to grant, and the second the emphasizes not only that right, but the necessity of the supplicant to continue to approach for that favor. The emphasis is still on the king, and the king’s absolute right to grant that which is requested.