At the candidates' matches in 1971 [Petrosian and Korchnoi's] paths crossed again. It was already clear that whoever won would have to face Fischer, who was swiftly ascending to the chess throne. There was practically no doubt that Spassky would deal with him, but our sports committee decided that that it was better to stop him on his march. Petrosian and Korchnoi were summoned and bluntly asked which of them had the greater chance against Fischer. Korchnoi replied that in the "Fischer age" almost no one had a chance, but Petrosian said that he believed in himself. At that Korchnoi was asked to throw the match to Petrosian, in compensation for which he would be sent to the three biggest international tournaments (for a Soviet chess player at that time this was a regal present).
No documents exist to substantiate this plot. But the mediocrity of Korchnoi's play and the fact that, considering his bitter nature, after he lost to Petrosian he remained on good terms with him implies that Korchnoi let Petrosian win. ['Karpov on Karpov', p.114]
Keeping in mind that the world's top chess players, especially the players who developed in the Soviet Union, rarely have anything complimentary to say about each other, the accusation that 'Korchnoi was asked to throw the match' and the implication that he acquiesced, are still stunning. Here's what Korchnoi had to say.
The match turned out be highly tedious; we played eight draws in a row! [...] People joked that neither of us wanted to win the match, and then meet Fischer. In the West many were thinking the same way, being unable to believe that the match was being played seriously. And only those who knew me well realized that I was trying very hard, but that my play was not coming off. I was most upset when, in the heat of the moment, I overreached myself, and lost from an excellent position in the ninth game. [...]
It was not difficult to guess that the last game of our match would finish in a draw, and Petrosian went through to meet Fischer. By his play against Huebner and me [Huebner abandoned the quarterfinal match], he did not deserve a place in the final candidates' match. But only Fischer was able to demonstrate this. ['Chess Is My Life', p.79]
What about the three international tournaments? According to my page on 1970-72 Candidates Matches, the Petrosian - Korchnoi match was played in July 1971. In the next chapter of his autobiography, Korchnoi mentions playing in the Alekhine Memorial (Moscow, autumn 1971, where Karpov also played), at Hastings (year-end 1971, again with Karpov), at Amsterdam (summer 1972), and at Majorca (November 1972). That corroborates Karpov's story. As for remaining on 'good terms' with Petrosian, Korchnoi's account of the semifinal match continues with the following.
After winning the match against me, Petrosian persuaded me to take part in his preparations for Fischer. For two weeks I visited his ostentatious villa on the outskirts of Moscow. Before his departure for Buenos Aires, Petrosian insisted that I should also go. The question was debated in the Sports Committee. I said that I was a participant in the same candidate's cycle, and so it was unethical for me to be a second, but I could agree if Fischer were to allow me. And I said further that it wasn't always pleasant for me to watch Petrosian's play to say nothing of carrying responsibility for it. In the Committee they did not insist, evidently realizing that the devil himself wouldn't help Petrosian against Fischer!
Karpov continued his account with:
But the idyll [that the players 'remained on good terms'] could not last long. Petrosian had a notorious appetite, and he didn't want to depart from his habits here. Korchnoi knew Fischer well, and in general knew a great deal, so why not make use of this knowledge in the match with Fischer?
This incident is known to me from Korchnoi's own account, although it generally received wide publicity in the chess world. After hearing out the request, Korchnoi could not contain himself and burst out laughing. "Now how the hell can I be Petrosian's second if it makes me sick to watch how he plays?"
This was the end of it. This wasn't just an explosion, but a challenge, and Petrosian vowed to annihilate Korchnoi. And now he was trying to do it with my hands.
That last sentence puts Karpov's account into context: Petrosian offered the young grandmaster valuable advice during the 1974 Karpov - Korchnoi final candidates match, which was a de facto World Championship match.
What do other sources say about the 1971 Petrosian - Korchnoi match? Kasparov glosses over the match both in the Petrosian chapter of 'Predecessors III' (p.108, not even a complete sentence) and the Korchnoi chapter of 'Predecessors V' (p.73, a paragraph)
Plisetsky and Voronkov's 'Russians vs. Fischer' (p.220) quotes exactly the same Karpov passage that I used in this post, with some rewording; Korchnoi's 'it makes me sick to watch how he plays' becomes 'his playing puts me to sleep'. The next chapter is a transcript of documents dated June 1971 that analyze Fischer's 6-0 whitewash of Taimanov in May of that year. That's followed by a long chapter, apparently another copy of an official document, by four Soviet GMs analyzing Fischer's play. It mentions the Fischer - Larsen semifinal match, played in July like the Petrosian - Korchnoi semifinal, leading me to assume that Petrosian had already qualified to meet Fischer in September.
Conclusion: the silence from the two other sources is deafening. Only Korchnoi's account -- 'those who knew me well realized that I was trying very hard, but that my play was not coming off' -- speaks in favor of a match that had not been decided by fiat.