If the search query only returns a single post, redirect to that post.

redirect-single-post.php

<?php

/**
 * If the search query only returns a single post, redirect to that post.
 * 
 * @return void
 * @author Paulund
 * @link   http://www.paulund.co.uk/redirect-search-results-return-one-post
 */
function redirect_single_post() {
  if ( ! is_search() )
    return;
    
  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;
  }
  
}
add_action('template_redirect', 'redirect_single_post');