Prayer is essential in the Christian's life. Without it, your witness will be far less effective, and you will be far more vulnerable to the enemy. When you witness, you need the blessing and support of the Lord. You need to be in fellowship with Him.
In prayer you ask God to give that growth. In prayer you ask God to convict the unrepentant of their sin and by that awaken in them the need for salvation.
Jesus prayed frequently (Matt. 14:23; 26:36; Mark 6:46; Luke 5:16; John 17).
Another reason to pray is that you can actually influence God with your prayers. If you are doubtful, then look at 2 Kings 20:1-7. King Hezekiah was told by the prophet Amoz that he should set his house in order because he was surely going to die (v. 1). Hezekiah prayed earnestly (v. 2,3). The Lord heard his prayers and said, "I have heard your prayer, I have seen your tears; behold, I will heal you. On the third day you shall go up to the house of the LORD. And I will add fifteen years to your life" (v. 5). Hezekiah's prayer made a difference.
Pray for more people to witness. Jesus specifically asked you to pray to the Father and ask Him to send workers into the field (Matt. 9:37-38). What is the field? It is the world of sinners.
Pray to the Lord to bind Satan and his angels. There is a hierarchy of demons seeking to hinder your witness and steal the seeds of the Gospel that you plant. You cannot fight spirits with reason or flesh and blood, but you can ask the Lord to fight. With prayer you can assault the camp of the enemy and weaken his false kingdom.
Prayer is a mighty tool, a powerful tool. You need it if you are going to witness.
Pray for your needs. Do you have a close walk with God? Do you need a deeper fellowship with Him? Do you have sins you need to confess and forsake? If so, then pray. Enjoy your privilege of coming to the Creator of the universe who meets your every need. He loves you. He wants to hear from you and He wants you to make your needs known to Him.
"Be anxious for nothing, but in everything by prayer and supplication with thanksgiving let your requests be made known to God" (Phil. 4:6).