I know that it is getting very very tough for India. History and facts are all stacked very heavily against India for a 500+ target has never, read NEVER has been chased down in test history. But then, there are some bright patches of light in this history for India as we chased down 400+ against West Indies at Port of Spain way back in the 1970s, which was the then highest test chase and also in the 1998 Independence cup match against Pakistan at Dhaka, we chased down 316 to win against Pakistan, being the then highest record chase in ODIs.
Pakistan, currently lead by 518 and the good point for India is that it has become a flat track now and a good pitch for batting. Considering Indias good record at flat tracks with the bat despite being under pressure in the 1st 2 tests, India, are still in with a chance to win this test match, but only slight. If India, manage a all-out taking the last 5 wickets quickly before lunch and are set somthing like 600 in 160 overs to win, then it all comes down to the batting order and Indias run-rate in the 4th innings which will decide the chances of an Indian win if it is possible.
For chasing a 600 to win this test match, we need a flying start to the innings by Sehwag, with Irfan Pathan or Dhoni being promoted up the order in the hope of getting quick runs. Also, One out of Dravid, Laxman or Tendulkar will have to play a real big one. We, did manage a 603 on a flat track last match despite being under-pressure, so we do have the batting to pull off a thrilling 600 run chase on a flat track in this match.