In my previous project i was using window.showModalDialog to open a modal window (ASPX Page) from parent ASPX page using JavaScript. But whenever the postback occurs in the modal window during <save > button click, it would spawn off a new window. When searching turned up my Colleagues with same problem who managed to work with window.open.Finally i found a solution which is very simple by just including below tag line in the <head> html of the modal window.
<base target=_self>
I hope this would be helpful for someone with the same problem.