Prueba con esto:
Código HTML:
<html>
<head>
<title>Untitled Document</title>
<script language="javascript">
function setCombo(valor)
{
document.getElementById("dias").selectedIndex = valor;
}
</script>
</head>
<body>
<form name="formulario">
<input type="text" name="texto" onKeyUp="setCombo(this.value)">
<select name="combo" id="dias" size="1">
<option></option>
<option value="1">Lunes</option>
<option value="2">Martes</option>
<option value="3">Miercoles</option>
<option value="4">Jueves</option>
<option value="5">Viernes</option>
<option value="6">Sabado</option>
<option value="7">Domingo</option>
</select>
</form>
</body>
</html>