Instead, 7...cxd4! would have assisted in establishing full equality for Black. Stockfish gives the following,

7...cxd4! 8. Nxd4 Nc6 9. Nxc6 Bxc6 10. Bd2 Ne7 11. O-O-O

Analysis Diagram - Position after 11. O-O-O

click for larger view

A) 11...Ng6 12. Kb1 Be7 13. h4 Bxh4 14. Ne4 O-O 15. g3 Be7 16. f4 Qc7 17. Bc3 Rfd8 18. Bg2 Rac8 19. Qh5 h6 20. Rh2 Bxe4+ 21. Bxe4 Qxc4 22. Bxg6 fxg6 23. Qxg6 <0.00/23>

B) 11...Rc8 12. Kb1 Qc7 13. Re1 Nf5 14. Qg4 Bc5 15. Bd3 O-O 16. f3 g6 17. Qh3 f6 18. Bxf5 exf5 19. exf6 Rxf6 20. Qh4 Re6 21. Bf4 Qb7 22. Rxe6 dxe6 <+0.04/23>

Aside from the above lines, Deep Fritz 13's opening book provides Portisch vs A Matanovic, 1965 as the earliest known game in which Black plays 7...cxd4.

Not only was this game played between two famous Grandmasters in 1965, but it was also played in Yerevan. Based on this, it is surprising that the 9th World Champion didn't go in for 7...cxd4.

One possibility is that he missed something in his preparations on this line and thought that GM Korchnoi would have an improvement ready for it - thus deciding to try something new and different OTB.

I didn't think much of <14...g6>. Surely trying to catch up with development (e.g. <14...Nf6>) was more important than creating a few empty threats on h6.

Quite, in my opinion also-here's something I posted elsewhere a few years back.

While 7...cxd4 probably would've been better there are reasons why Petrosian would have passed.

Opening up the position with complications from 8. Nxd4 Nc6 9. N4b5 with maybe 9...a6 10. Nd6+ Bxd6 11. exd6 probably wouldn't be his preferred style w/ Black against Korchnoi whose strength is putting difficult problems out in complicated positions. Rather than face a bunch of tactical possibilites Petrosian probably hoped to simplify and hold the endgame.

It should be noted that, according to Stockfish, 7...cxd4! is indeed better (and not merely probably better).

Of course there is no way that the 9th WC and his team would have had recourse to Super-GM level Chess engines in the '70s, but it is unfortunate that they did not give the lines starting with 7...cxd4! more serious consideration - as they, at least on one notable occasion, had already proven successful.

LTJ

http://www.chessdryad.com/articles/...

But he did recover to win the next game in good style:

Petrosian vs Korchnoi, 1974

In fact, an oddity of the 1974 Candidates is that it seems that in several cases the best game of the match was won by the loser.

In fact, come to think of it, in my opinion, in the 5 matches in which the loser won a game at all, the loser had the best win every time. IMO, this happened in:

I'm pretty sure Petrosian's health or related had something to due to his poor form in the match. It seems in general he wasn't playing at his usual even from game 1 and it was reported that after Korchnoi's request for postponement of game 6, Petrosian was in the hospital for kidney pains and had to resign.

I think Petrosian's poor form showed even in his best game 4, Gligoric noted that Korchnoi in a poor line and serious time trouble still had drawing chances.