AutoComplete inside keyup event not working properly

Autocomplete list doesn't shows up when first key is entered. The list is shown only after entering the next key. Sample code is given below

$("#AutoComplete").on('keyup', function (event)

{

if ($("#AutoComplete").val().length <= 5 )

{

GetData();

}

});

function GetData()

{

$.ajax({

type: "POST",

url: "../Controller/function",

contentType: "application/json; charset=utf-8",

data: '{"key":"' + $("#AutoComplete").val() + '"}',

dataType: "json",

success: function (data)

{

var output= $.map(data, function (item)

{

return {

label: item.Name,

value: item.ID

};

});

$("#AutoComplete").autocomplete({

source: output,

autoFocus: true,

});

}

});

}