Take a good look at your shoes. they are not made equal. I find Nike and some Wilson shoes tend to slope down from heel to toe, give extra stress to the knee.
I was wearing some Wilson shoes which look very good but my knees hurt so much I could barely walk down stairs. I had to switch back to my badly beaten Addidas shoes and the pain was gone in a week.
Generally speaking the sloping is quite common in running shoes. But it's a sin to have it in tennis shoes. Most of the stress put on the knees is not due to the running, but the constant "go and stop" in tennis, and the shoes should be perfectly flat from heel to toe.