add_action('template_redirect', 'redirect_single_post');
function redirect_single_post() {
if (is_search()) {
global $wp_query;
if ($wp_query->post_count == 1 && $wp_query->max_num_pages == 1) {
wp_redirect( get_permalink( $wp_query->posts['0']->ID ) );
exit;
}
}
}