rajeshmarndi, Often, I think, it is not fruitful to retrace history and ask how someone was led to a theory a century ago. It would not, for example, help us understand Maxwell's Equations any better to read about the experiments done by Benjamin Franklin. I think we have ways now of understanding relativity that are better than what Einstein himself had, at least what he started out with.
Anyway, the principle that led to General Relativity was the Principle of Equivalence, the idea that the universality of gravitation implied that it was a property of space itself and not of particular objects. Mercury's orbit was an early prediction and an important confirmation, but is not what led to the idea. (Could you imagine taking a 43"/century discrepancy in the orbit and deduce from that the idea that space is curved?)