Thanks Ernesto for the suggestion.
I have implemented as mentioned by you and this worked fine when i filled all the form fields and submitted the Form. Once submittion is complete, i navigated to Form library where the form gets submitted and opened the item and found that the Submit button is disabled.
But, if i leave the mandatory fields empty and click on Submit button, a validation error message would trigger and once i close the validation message and come back to form, i found that the Submit button is disabled. And i understood why it came because the rules were set in in Submit button control and upon clicking it the rules will any way execute.
So what i did is I set rule when the form loads to set the DisableSubmit field to TRUE. For settings this, follow the below steps:
Go to Tools--> Form Options --> Select Open and Save --> Under Open Behaviour, select Rules.
Under rules, Created an action for setting the DisableSubmit button equals TRUE when any of the mandatory fields in the form are not blank. As we know that when the form loads for the first time, this condition doesnot satisfy and thus DisableSubmit as set by default will be TRUE. Once we submit form, all the mandatory fields will anyhow have values and in second loading of the submitted form will satisfy the condition and then the DisableSubmit will be set as TRUE.
That is, Assume Location is a dropdown field in my form which is mandatory then
If Location != blank, Set DisableSubmit=TRUE
Here by my issue was fixed.