2004 a8l not cranking all the time

My 2004 A8l 4.2 has been getting worse it will go 2 weeks with no problem then act up . turn key hear a good click and no crank can take up to 15 min of trying to get it to crank then good for day or 2 . or will start with 1 or 2 trys been going on for about a year . but getting worse . Starter?...Starter relay ?.... other ? Thanks for any input