add_action( 'init', 'wpse9870_init_external' );
function wpse9870_init_external()
{
    global $wp_rewrite;
    $plugin_url = plugins_url( 'api.php', __FILE__ );
    $plugin_url = substr( $plugin_url, strlen( home_url() ) + 1 );
    $wp_rewrite->add_external_rule( 'api.php$', $plugin_url );
}
add_action( 'init', 'wpse9870_init_internal' );
function wpse9870_init_internal()
{
    add_rewrite_rule( 'api.php$', 'index.php?wpse9870_api=1', 'top' );
}
add_filter( 'query_vars', 'wpse9870_query_vars' );
function wpse9870_query_vars( $query_vars )
{
    $query_vars[] = 'wpse9870_api';
    return $query_vars;
}
add_action( 'parse_request', 'wpse9870_parse_request' );
function wpse9870_parse_request( &$wp )
{
    if ( array_key_exists( 'wpse9870_api', $wp->query_vars ) ) {
        include 'api.php';
        exit();
    }
    return;
}

if(strpos($_SERVER['REQUEST_URI'], "index.php?wpse9870_api=1"))
{
   header("HTTP/1.1 200 OK");
   echo "v1p";
}

$.ajax({
            type: 'POST',
            url: '/index.php?wpse9870_api=1',

{"readyState":4,"responseText":"v1p","status":500,"statusText":"Internal Server Error"}