jQuery UI - autocomplete error

jQuery UI - autocomplete error

I'm using jquery.ui.autocomplete and datepicker for an implementation in
my webpage.
As separate file
jquery.ui.datePicker.js
jquery.ui.autocomplete.js
With the following simple code
var availableTags = ["ActionScript", "AppleScript", "Asp", "BASIC", "C",
"C++",
"Clojure", "COBOL", "ColdFusion", "Erlang", "Fortran", "Groovy", "Haskell",
"Java", "JavaScript", "Lisp", "Perl", "PHP", "Python", "Ruby", "Scala",
"Scheme"];
$("#City").autocomplete({
source: availableTags
});
However I'm not able to get it working in webpage as expected, whereas
datepicker is working great. Sorry, I couldn't reproduce this behavior in
jsfiddle.
Error from console: pointing the jquery.ui.autocomplete.js file
Uncaught TypeError: Object [object Object] has no method 'menu'
jquery.ui.autocomplete.js:185
I've imported both js and css properly. Please share your suggestions
jQuery version: 1.8.2
jQuery UI version: 1.9.2