If Fischer would played Karpov for the World Champion, who would win?

Hard to say. I think Karpov was better prepared for the match, so I would give him an edge.

I do agree that Fischer would eventually beat Karpov in a match where the first player to win 10 games is the champion. But the match would take well over a year, and Karpov would lose due to fatigue rather than chess skill. 

Karpov went entire years where he lost five games or fewer, and he was the most active world champion of the 20th Century!

Karpov was 8 years younger than Fischer.  Seems doubtful a 24 year old Karpov would fatigue first.

It does seem unlikely based on age, but Karpov was not physically strong and grew tired in matches even in the prime of his chess career. 

I think it's very realistic that Karpov would've been Fischer.  Not saying that for sure... But I think he could've done it.  I think Kasparov would've definitely beaten Fischer.  

Strange how many people on here just seem to think brains turn to mush in their 30s.  Everything depends on how you take care of yourself.  Anand is killing it right now... He beat Magnus recently.  Try not to be so ageist.  

It's possible to go even further and say Fischer was legitimately intimidated by Karpov... 

Fischer beat Spassky... But Spassky was afraid of Karpov.  Spassky knew Karpov was better than he was.

It's safe to say that Karpov was unlike ANY opponent Fischer ever faced.  Until Kasparov came along, it can be argued Karpov was the best Chess player ever up until that point, in terms of sheer ability and talent.

Karpov

Hard question. If Fischer played his best, I think he would win but I believe Karpov's style would prove more difficult than Spassky's for Fischer in particular. Karpov seems more solid and that doesn't work well for Fischer's aggressive attacking so he might struggle but I still think he'd break Karpov's wall as he did Petrosian's and take home the win.

At the time, Karpov was built like a teenage girl. I met him.

It would have been an interesting battle, but my money (paltry then) would have been on Karpov.

Knowing what we know now, that Fischer had tried quitting chess before, during, and after the match with Spassky (and partially retired even before that) it seems pretty clear he spent his 3 years not even considering professional chess... meaning he was very rusty.

Had Fischer stayed motivated and healthy, then probably Karpov would have lost.

Fischer was never healthy

Yeah, and I liked Kramnik's approach in the interview where he assessed past world champions when he said he doesn't like speculating about separating the good from the bad because that would be talking about a different person. Part of what made Fischer himself was his issues.

fisher refuses to play with karpov back in his time. So for me Karpov

Donner was right, he wrote before the match 1972:

"... it doesn't take much insight into human nature to predict that Fischer will not be world champion for long. His quirks, moods and whims will turn against him at the moment when he has reached the top. He'll hit out hard, but at nothing but thin air."

BL4D, thanks for bringing us that quote.  Donner was a smart fellow (and tall--judging from photos--I never met him).  "Insight into human nature" is, however, not as simple as it sounds.  Fischer had drive and, if not education, intelligence of a certain kind that leads to achievement few people have reached.  Quirks, moods, and whims--are they driving or limiting?

Piaget and Erickson formulated insights into how children grow into functional adults, and they apply to chess development too.  Fischer and Karpov grew up in quite different worlds, and, of course, they developed chess playing on two different genetically programmed brain machines.  Their prehension of chess calculation and positional mapping were genius level, but not the same.  Their creativity applied to different kinds of problem solving--position and tactic solutions.  That is what fascinates us about the match that never was.

But, why do we care now?  Children today grow up with computer assisted, high impact teaching methods neither Karpov nor Fischer had access to.  The world of 1975 is gone and will never be reproduced.  Tomorrow's World Champions will be stronger players.

Fischer was Better than Karpov, that doesn't mean he would have beat Karpov. This may seem counter intuitive but all we have to do is look at Kramnik vs Kasparov. No one argues that Ktamnnik was better than Kaxparov, Kaspariv is the best chess player of all time. Sometimes the better chess player loses to a specific player of inferior strength. No one argues that Petrosian was inferior than Portisch though Petrosian had a losing record against Portisch. Petrosiab had a losing record against Korchnoi even though no one thinks Korchnoia was a superior chess player. Long story short, Fischer could have lost to Karpov even though he was better in 1875.