This was really good comeback. Given that he had to deal with Mayer being not able to walk in 4th set to all of the sudden serving well and moving well in 5th, it wasn't easy by any means.
In third set, after Richard lost a break up lead, he looked defeated. I really though there is no way he could win, especially that it is on clay and match can go to 5 hour mark if he wants to battle in third set. What happend in third was Richard kept fighting, although honestly playing very average tennis at best. He was missing returns, missing easy forehands, was hitting his backhands short, etc. Mayer choked pretty badly in third set and was not there in 4th, but 5th set Richard played fairly well. This is only second time he comes back from 0-2 down, but it was huge.
Bravo Richard. Great fight.
Under a similar scenario he lost to Youzhny 6-8 in the 5th set 2007. Youzhny could not move either for large parts of the match, but Richard could not deal with it. I totally agree with you, this was a huge win, even if he did not play his best tennis. Confidence-wise it might be much more important on the mid-term than a straight sets victory.