He retired because of blisters, but at that stage it was already clear that he wasnt going to trouble Andre. He said after the match that the blisters werent so bad and if he was playing a lesser player that day he would have continued, but against Andre he had no chance if he wasnt 100%.
Coria is a fit man, you cant use his last match as an excuse in advance if he loses. The match was 4.5 hrs, but there was a day in between, so they had a chance to get plenty of rest before they resumed. I think Andre has a better chance to win, long match or not. Andre killed him in their previous meetings. Clay is obviously a better surface for Coria, but Andre is no slouch on it either. The match will be close, the only way i can see Coria winning is if he gets enough balls back to frustrate Agassi into going for too much.
I don't know if anyone here saw Coria's post match interview.
He basically said that when Agassi was playing well, he was a step above everyone else. He said that Andre was the player whose game he admired the most. When asked how he would play against Agassi, he said that it would depend on what tactic he thought he needed on the day, but added that idealy he wanted to play like Agassi.
Although he also said he was anxious to play Agassi on clay, I don't think these are the remarks of a player who thinks he can win. Hope I am wrong!