We propose a novel user interface that enables control of a singing voice synthesizer at a live improvisational performance. The user first registers the lyrics of a song with the system before performance, and the system builds a probabilistic model that models the possible jumps within the lyrics. During performance, the user simultaneously inputs the lyrics of a song with the left hand using a vowel keyboard and the melodies with the right hand using a standard musical keyboard. Our system searches for a portion of the registered lyrics whose vowel sequence matches the current user input using the probabilistic model, and sends the matched lyrics to the singing voice synthesizer. The vowel input keys are mapped onto a standard musical keyboard, enabling experienced keyboard players to learn the system from a standard musical score. We examine the feasibility of the system through a series of evaluations and user studies.