I am currently read "Tournament" by David Crouch, which is turning out to be a pretty enjoyable read. However, in it he asserts that,

Tournament by David Crouch, Pg.16 wrote:

...we know for a fact that a twelfth- and thirteenth-century knight was physically able to tourney only for a day[rather than 4 days].

Given that many knights trained daily in some form, would this assumption be true or could a tourneying knight have fought in a multiday tournament?

I accept the question of stamina required to fight several days as valid. Given that opponents were free to retreat to resting zones (recets) in 12th / 13th century melee tournaments, this is probably not the primary point of his statement. The Grand Melee event was generally the final event of a multi day tournament for a specific reason.

I have several of Mr. Crouch's books. In context based on reading all of them as well as several others, what is really being pointed about here, by him and other authors as well, is that a full day of 12th/13th century melee fighting with serious intent to win resulted in a lot of extremely painful bruises. Although they probably did rebate the edges on their swords, one can imagine the effects of repeated blows against mail. Most instances of death in tournament were not from the fluke "pierced with a lance" or "cut with a sword", but by "complications from bruises" or being trampled under horse. The "fortnight between tournaments" has also been explained (with historical texts as a a basis) as a necessary period of recuperation from bruises recieved in one hard day of melee contest. Contestants were sometimes characterized as unable to even ride their horse for period of a couple of days after a day of melee tournament.

That makes good sense. I think the context he was talking about at that point he was comparing a knights realistic stamina to that of what some heralds had suggested their patrons could fight for.

In that case, his suggested comparison of a recovery time being a little longer than for professional rugby would be about right, due to the amounts of bruises and other niggling injuries that could occur.

Having personal experience of the effects of repeated blows against mail delivered by enthuseastic friends in training i can testify to the bruising and effects. Anyone who watched the program on William Marshal will have winced as I did when they heard about Williams helmet being crushed down to his scalp by a blow. A blow that hard even in a great helm would have hurt not least as the full force would have been taken on the shoulders even with padding that has got to sting. No sword would have crushed a helm like that only a mace or axe would have delivered a strike like that a blow of that power against mail would have caused serious injury even with padding. Given blows of that magnitude it is not surprising men could not ride for days in fact the question should be how the death toll was so low.

On the matter of knights fighting all day there are numerous acounts of day long battles. Crusaders fought for hours in desert conditions not continuous battle but still mentally and physically exhausting. It is well documented that if the first charge failed assaults tended to have a rhythm pulses troops surging in fighting then breaking off to regroup and rest then attack again. These pulses were determined by the physical fitness of the troops involved also moral of the troops the side which cracked first lost. I have fought in and marshaled re-enactment battle over thirty years the trick is judging when to pull back and rest in any line you will have men of different fitness levels. The key is to push the lesser men but stay below the best I believe medieval commanders were aware of this and where possible made sure men got a chance to rest.

Good point I would have to question the intention of the man who delivered the blow to Williams helmet. I suspect enthuseiasm got the better of him that time it happens I have the odd ache to prove it.