Everybody has the same problem — even the professionals. For nine days Lendl plays so well. Then on the tenth day, he plays badly and loses. Since you are following the spiritual life, you have to depend more on divine Grace than on your own capacity. On the day when you have a match, many hostile forces may attack you. Either doubt comes and you feel, "I may not be able to defeat so and so," or overconfidence comes and you say, "Yesterday I played so well, so today also I will easily be able to defeat my opponent."
What you have to do for a few minutes is pray and meditate with the idea, "By Your Grace, O my Supreme, I will play my best." You have to depend on His Grace, rather than on your own capacity, because even our own capacity is His Grace. So you have to depend on Him to give you the capacity to fight against doubt; you have to depend on Him to purify and illumine your consciousness so that you do not have nervousness, overconfidence or anything else.
Again, there is an even higher prayer: "By Your Grace I will play the way You want me to play, and I will be equally happy no matter who wins." Unconditional surrender is the highest prayer. Second best is to pray to play extremely well through the Supreme's Grace. Let us not bring unconditional surrender into the picture right now. If you use the second best prayer, then God's Grace will enter into your nervousness, insecurity, doubt and other wrong forces and you will always be able to maintain your regular standard.