Mark Webber has admitted fading motivation prompted his decision to retire from Formula 1.The 2013 US and Brazilian GPs will be the veteran's final grands prix in a career spanning more than a decade, with 9 wins and 40 podiums.Webber (37) is switching to Le Mans sports cars, to head Porsche's new prototype foray.'I WASN'T READY'He admits he was already starting to think about retirement in 2012: "Porsche wanted me for 2013 but I said that I wasn't ready yet. During that year (2012) I started to think about a change. "I had already been at Red Bull for a very long time, so you do think about doing something else. That's human nature."Webber admits he courted a move to Ferrari, but ultimately decided last Christmas to quit F1. Another factor, he said, was his fitness.Webber said: "A lot of people have trained with me but most are gone after less than two years. I never had to be motivated to keep fit."But over the past year the urge went away. So I had to ask myself why. Then it becomes clear - 'Mark, you're not 19 anymore!'He admitted that, increasingly, the desire to do things other than F1 crept in.Webber said: "In my case, the reasons are very personal. I want to spend more time with my family, my partner, my friends. Suddenly there are things on the radar that weren't there before."I had often heard other sportsmen and woman talk about the problem of their motivation going away, but I always thought to myself, 'What?'"The fact is that it does go away. Also, when you're 36, you think differently to when you're 25, and that doesn't just go for drivers. As an athlete, you put so much into your career. I wouldn't say that we make sacrifices, but if you start thinking that, then maybe it is time to do something else. You have to believe that it's all worth it.'IT'S PRETTY EXHAUSTING'"I'm enjoying the summer in Australia and then suddenly you have to go to Jerez for winter testing and you think, 'Hmm...'"I also knew that I wouldn't be happy if I stopped racing completely. You have to find a balance, something that stimulates you and I've found that with Porsche. I know we have a lot of hard work ahead of us, but it will not be like formula one. Next year there will be 20 grands prix again and if you're in a top team, it's pretty exhausting." "I want a different balance in my life and the timing is right, and that (finding the right time to retire) is not easy for an athlete. Look at Roger Federer, look at Valentino Rossi. I have the feeling I'm going pretty well at the moment, even if I don't have the results to prove it, for reasons that we both know."