BERLIN, Germany - It is the nature of modern F1 that has contributed to Michael Schumacher's difficult comeback so far, the seven-times Wworld champion has insisted.In 2010 Schumacher announced his return after a three-year break to the sport he once dominated yet, despite an incredible 91 wins and 154 podiums in his first career, he hasn't reached the podium in 38 Grands Prix with Mercedes since 2010.It has been speculated that Schumacher is now either too old to perform or no longer focused and determined. He said, "Some have questioned whether my mood has changed. It would be nice if outsiders didn't make guesses without knowing."MAJOR CHALLENGESchumacher insisted that F1 was a "major challenge" both for himself and for Mercedes, which is still building up its works team.He said: "I thought that I might be (on the podium) at least a couple of times. Together we have to work hard to improve performance but we are not afraid of that. In F1 there are no magic wands."In the past we did a lot of testing. Today not, so I have had some problems with the tyres. I have not learned how to interpret them correctly. Previously I came to the race weekend knowing what was coming. Now the tests don't come until Friday and I am still getting used to this system," he said. Schumacher's existing contract runs out at the end of 2012."Will I be here in 2013? There is no hurry yet to think about it," he said.